I have ridden a few Alsa buses in the past month.
(1) first if you know the bus you want buy the ticket ahead on the Alsa app, (which is much better than the Renfe app). Tickets do sell out as I have learned much to my chagrin. Set up an Alsa account and then you can put your ticket in your Apple wallet. Also, in many cases, if you buy on the app, you can pay an extra € or so to select your seat and not be in the last row. I like a window seat toward the front of the bus so I can enjoy the scenery.
(2). My 36 L backpack goes underneath with everyone else’s luggage – lots of shopping, suitcases, etc. I keep my Crossbody/day bag with me with all the important stuff. I have never worried about my backpack in the bowels of an Alsa or other bus.