Air Canada to Athens and then Sky Express to Santorini

So assuming that your flight is Toronto to Athens, you will clear immigration and customs in Athens. It sounds to me like you have booked the next flight as a separate flight. Please please please allow a lot of time. Pre-Covid I allowed 4 hours now I’m allowing even more

You will pass customs and immigration controls at Athens airport since this is where you will enter the Schengen area.

The building is divided into two zones, the intra-Schengen and extra-Schengen zones.
Whatever the area, the arrivals are on the ground floor, the departure area is on the floor above. All you have to do after clearing the controls is to go upstairs to check in for your Athens Santorini flight.

Athens airport is a relatively large but very easy to use single building. What is complicated are the huge queues of travelers during the summer. I was at Athens airport 2 times this month and in the morning it is very crowded to check in baggages. So it's a good thing you don't have checked baggage, you'll save about an hour.

However, to be safe, plan a minimum connection time of 2h30 to 3 hours

You will no longer have any control in Santorini and can calmly think about your birthday cake.
Be careful, Greek cakes are very very sweet!

PS: there is also a kind of satellite terminal which is linked to this main building by an underground tunnel with conveyor belts. If you have to go through this satellite, which is unlikely, it will only be a few minutes in an underground corridor.

We just returned from Greece. Flew Air Canada from Toronto to Athens. We had a 45 minute time frame to get to our flight to Crete with Aegean airlines.
On arrival, Air Canada announced that everyone not making a connection should stay in their seats and allow everyone making a connection to get off first. They did check our boarding pass and sent a few people not connecting back to their seats. We managed to get to luggage pick up very quickly Just follow the signs.
We then went to passport control and got through in about 10 minutes. There can be up to 4 jumbo jets arriving at the same time so not having to wait for baggage will allow you to get to Passport control quickly. (The AC direct flight arrived around 10:30 AM and the airport was not really busy.) We then left arrivals level and took the escalator up to departures. There are also elevators and stairs.

We rushed to the Aegean check in desk, and dropped off our bags then headed to security. Athens airport has a well organized security process and it took us no more than 5 minutes to get through security. We were on the aircraft 15 minutes before departure.
You should not have a problem particularly with hand luggage.

The satellite terminal is used by some European airlines. but not the major airlines. You do not have to be concerned about that issue AC uses the main terminal.

Stanbr, u r back! We expect pictures.

Got thousands of photos. Will start to post them over the next week or so.

Oh I’m hoping this holds true today! Our Air Canada flight is over two hours delayed, meaning our 3 hour connection time is now barely an hour. Our Santorini flight is on Sky Express, not Agaean, and I’ve been spending the past hour trying to message Sky Express from the plane to see about reserving seats on the next flight. But maybe we’ll get lucky and AC will help us out.