Trains and buses within Bavaria?

The Bayern ticket costs 27 EUR for the first person and 9 EUR for each additional person, up to 5 people traveling together, after 9 am weekdays, anytime weekends. Valid for all regional trains, and public transport of all means in the state of Bavaria for the day. Includes transport to/from Salzburg on regional trains. Buy them out of vending machines on day of travel. Nothing requires advance purchase.
https://www.bahn.com/en/offers/regional/regional-day-ticket-for-bavaria?dbkanal_007=teaserSliderBigImage_3-1_link_laenderticketbayern

https://www.bahn.com/en
This is probably the most useful website for your planning. There are various passes and tickets.

This website below has a lot of useful information that tells about different types of passes and tickets. I would spend some time reading through this information.
https://www.seat61.com/train-travel-in-germany.htm

Are you arriving at Munich’s airport? If so, go straight to Garmisch-Partenkirchen from the airport by hopping on a direct bus. Busses operate every two hours taking 2h 15m.
I’m assuming you are going to Zugspitze while in Garmisch-Partenkirchen. From Garmisch-Partenkirchen take a train to Zugspitze Bahnhof Zugspitzplatt (1h 15m) then hop on the cogwheel train to the summit. When you are ready to move on rent a car in Garmisch-Partenkirchen.

I’m a little worried about the post above… please double check on Zugspitze directions. I’m not sure it provided you with enough detailed instructions.

Also, no need to rent a car unless you want to.

Reservations for lodging were set up long ago...

OK, not sure why you've been instructed to travel to G-P on the same day you arrive. There's a direct TRAIN that will get you from the airport train station to central Munich, if that's where you are booked. Buy this ticket at the airport ticket machine for that trip:

https://www.mvv-muenchen.de/en/tickets-and-fares/tickets-daytickets/airportplus-ticket/index.html

For your other journeys, do as Sam suggested with the BAYERN TICKET.

For ALL your journeys, you can hunt down a travel schedule at this page; just enter your dates, to/from stations # of passengers... and click on "only local transport." This last step will ensure that you are looking only at schedules for regional trains (the only trains you can use with the Bayern Ticket.)

https://reiseauskunft.bahn.de/bin/query.exe

As Sam said, don't buy these tickets in advance. There's no discount for you if you do, and there are no refunds.

Buses: They are covered by the Bayern Ticket, but trains are possible for all your stated journeys. The only exception would be if construction is underway somewhere and a replacement bus (SEV) is required. Within Germany, city buses, trams, and subways are also covered by the Bayern Ticket (not so in Salzburg Austria, however.)

Is there a pass that covers it all?

Yes... the Deutschland Ticket costs €49/mo. each. However, it's not a simple matter to buy, and it involves a subscription which must be cancelled. I am pathetically underinformed on the details for this ticket, but you can search this forum for more information on it.

Thanks to all who have responded for your very helpful information! What a great resource this forum is.

I bought the Deutschland ticket last week. In fact I bought 3, one for each of our party. It was actually quite straightforward. Here is how I did it.

1. Have each person traveling download the MVV app onto their phone and register.
2. Use the MVV website for the actual purchase. Buy each person separately with their sign ins.
3. You will get occasional error messages. Simply hit the back arrow on your browser and click the appropriate page link again and the process continues.
4. You will receive an email confirmation. It is important to keep this email as it contains the link to cancel your subscription.
5. Your ticket will show up on your app
6. Be sure to cancel your subscription by the 10th of the month if you don’t want to be charged for the next month. I bought D-ticket for September one day and cancelled the subscription effective 9/30/23 the next.

The process is not that hard. The challenge is all the different (and often conflicting) directions out there on the web. If you are willing to put out a small amount of effort the savings can be substantial.

Buses: They are covered by the Bayern Ticket

Just to clarify: This does not apply to the Flixbus from MUC to Garmisch recommended above, which is not subject to the public tariff.

Do trains make all these connections?

There are train stations at the Munich airport, in Munich downtown, in Garmisch-Partenkirchen, in Salzburg, and in Regenburg, are they are all connected. There is no station in Schwangau, but there is a train station in Füssen, connected to all the above stations, and bus service between Füssen and Schwangau.

I've made lots of trips by regional trains and buses in over twenty years of travel in Germany, and never purchased (needed to purchase) tickets for these conveyances in advance.

Yes, the bus from the airport to Ga-P only takes 1H15, but the train takes less than 10 minutes longer and leaves from the Munich Hbf hourly, not every two hours, like the bus, so you will get there faster, on average, with the train.

Just completed a version of this trip a few days ago. The cog rail train/cable car to the top of Zugspitze costs EUR68 but the view is worth it. Our train from Munich to Garmisch involved a bus connection, I think because of track work. Check the DB app for details to see if your route is affected.

It looks like that track work ended on Sept 3. "EV" is for Ersatz Verkehr (substitute transport).

The Zugspitze cog rail train tunnels through the mountain to a station on the back side, from which you can ride a gondola to the summit. Or, you can get off the train at Eibsee and take a cable car directly (faster) to the summit.