Amsterdam 3-4N ? what are the highlights for first time visitor

You really need to get a guide book.

At the risk of sounding obtuse, why are you going? You've made the decision to visit those places and gone far enough to define an itinerary yet you're asking what to do there.

When I decide to visit somewhere it's because there's something there that I want to see or do.

I've been to Amsterdam once, for my stag do, and I'll put my neck out and suggest that the highlights of that trip would probably not be what you are looking for. What are you interested in? One person's highlight is another person's "to skip" so it's all subjective.

Ok this is all about personal interests.. and yours may vary.
I think three nights is enough in Amsterdam.. Ive been 4 times now.. and last two times I had someone who had never been before.. so the things we did were visit Anne Franks house. the Rik museum.. and wandered around and visited the coffee shops ..
When I was younger ( my first two trips) I did the visit the Wooden Shoe factory.. the Tulip gardens, the cheese markets , see the Windmills etc.. that kind of stuff does not interest me anymore ( and frankly some of it didnt interest me then but felt I had to do all the stuff )

Do you mean Brugges.. ? One night .. its cute but thats it for me ( are you into lace and chocolate shops.. ) Sorry I dont know any Burges

Brussels.. its been too many years for me to be any help.. I only recall it was a bit boring for me.. but I was younger at that time and perhaps sought something different.

Venice is magical.. 3 or 4 nights for sure. Florence is nice.. but for me .. 3 nights was sufficent.. but others swear they can enjoy a week there easily.. so thats just personal interests and tastes.

I dont know your age or tastes.. so my advice could be completely unsuitable for you.. if you are a huge history buff.. you will want longer in Florence.. and perhaps be able to find more to interest you in Brussels than I did..
Havent done Milan.

I agree you must do some more homework yourself. Your questions are too broad to give them a satisfying answer.
Get more informtion on what to expect before you decide how long to stay in your destinations, and you might find it even better to put in other places. Chances are that Brussels will disappoint you. The same is possibly also the case with Milan. You might find Gent a nicer place to stay than Bruges or Brussels, while it is only 30 mins from each. You could put in Bologna between Venice and Florence. Bologna is a lively place, and a good base for smaller cities around, such as Ravenna.
What »public transit« (in European English: public transport) do you mean? When you mean within Amsterdam, a map will show you the interesting part of the city is small enough to walk.
What kind of »rail pass« you have in mind? With your travel plans it will be a waste of money. Consult the trustworthy Man in Seat 61 for any question on trains, and how to book them in advance to get considerable discount. Also have a look on the pan-European train planner of the Deutsche Bahn to have an idea about the trains between your destinations.

Lots of time between now and October to consult some guide books. Failing that, google "things to see and do in _______" and wonderful information will come up. No one here knows your interests or even what you consider "highlights". One person's highlight is another person's boring waste of time.

Everyone's must-sees are different--there's the typical tourist sites and then there are personal choices. I spent 5 days in Amsterdam last April and found it to be extremely easy to navigate via public transit--buy a pass and you're set.

The one absolute must for me was visiting Anne Frank's house--her diary means so much to me that it was indeed like visiting the home of a dear friend. I also enjoyed the Dutch Resistance museum, and for art the Van Gogh, Stedelijk and Riksmuseum were musts. I also spent a day at Keukenhof as it was tulip season in April. On a personal level I enjoy flea markets and antique shops when traveling so went by ferry to the IJ Hallen flea market and also spent a few hours in an antique collective on Elandsgracht where I found some WWII fles porcelyne tiles. I stayed near the Vondelpark and wandered that in search of interesting birds. It's a lovely and unique city that's perfect for just wandering, so allow time for that, too.

To help get you started with your homework, go to this Explore Europe section of the RS website, choose your country, then choose the city of interest. Each choice will provide many links to help answer your questions.

None of your posts indicate your interests. As mentioned before, you need to invest some time in determining what the various cities have to offer and then decide what YOU want to see and experience in the various countries. There are numerous guide books available to purchase, or you can visit any public library and check them out for a few weeks. They will also provide you with options on transportation, hotels, tours, day trips etc. Get your hands on a few, start to do your homework and acquire some background knowledge. Then you can return to the forum with more specific questions that can be answered by fellow travelers.

I also have to ask, even though this is the Netherlands forum, why you'd want to travel from Venice to Florence and then back to Venice before going on to Milan. Plenty of ways to go from Florence direct to Milan. Or are you trying to decide between Florence and Milan after Venice?

Because I will take plane from Brussels to Venice, then train Venice-Florence-Milan, Milan to Vancouver ( fly).

I don’t know what’s the best route for this trip ?

For the train from Amsterdam to Brugge (Bruges) book via https://www.b-europe.com/EN. If you want to use the Thalys, you can book 4 months in advance. The sooner you book, the cheaper the tickets will be.

Train tickets from Brugge to Brussels can be bought at the station.

For public transport in/around Amsterdam there are different passes. Which one is best for you depends on how often and where you want to travel.

It depends on where you stay in Amsterdam what the best way is from Schiphol. It could be a train, but it could also be bus 397 or even maybe a taxi/uber. Train tickets from Schiphol to Amsterdam can be bought at the station for 5,30 euros or for 4.20 euros from https://www.discoverholland.com/Ticket/13/Schiphol-Airport-to-Amsterdam-train-ticket (which is a website from Dutch Railways).

The three fav things we did in Amsterdam were Anne Frank House, Van Gogh Museum, and the Heineken Tour Experience. We found the Rijksmuseum only so-so. Brussels is probably my least favorite city in Europe thus far, but really liked Bruges.

The answer to your question is on this very website! Click on "Explore Europe" in the above left hand corner, then "Netherlands" and then Amsterdam. After you read all that information look at all the other places you mentioned and then get back to us. Not to be too obvious but the owner of this website has a book or two you should really check out.