I've written this up assuming you didn't already have a PS5 (I didn't check):
I've done some quick tests with mine and here's the result:
You can start the PS5 by inserting a disc, or by pressing the PS Button on a controller/the remote. (You could also press the power button, but I never use that)
If you start the PS5 with the remote, the media tab of the menu will be open after account/profile selection.
If you start the PS5 using the disc, the last open application will be selected. Even if you connect the remote once prompted.
If a disc is inserted, while in the main menu, the disc will be highlighted, once it's read, if the focus/cursor is in the apps/games row. (You're not looking at news from a game or smth)
Otherwise, during playback, the remote is a controller which is missing some buttons and the joysticks. If you've got a friend with a PS5, I'd suggest figuring out the corresponding buttons on a controller and try to watch something using only those buttons and see if you miss some functionality.