Additionally, one can always swipe the bottom bar to switch between apps.
Eg Photos seems to change Back on almost every view. Swiping back navigates around albums and locations and such. Until you open a photo, then it flicks between individual photos. If you are looking at a photo and want to return to album view you need to click the Back arrow at the top left. If you swiped up to view photo details and want to go back to the photo, you need to swipe down. If you are Adjusting photo details, you need to click Cancel at the top left. If you are editing a photo and want to go back to the photo, you need to click Cancel, which is at the bottom left this time.
The only place I don't notice a back button in Photos is the top right. But that's exactly where Books puts it.
On Android, every single one of navigations would be performed by the dedicated back button/swipe. Which might explain why the iOS Google Photos app seems to just stick with a top-left back button on almost all views.
eg View details in Photos, and swiping back doesn't work. But there are also no back buttons or visual cues. But there are 3 ways to get out of that screen AFAIK, which some might label intuitive. But if you go one step deeper and Adjust the photo details, the only exit is a Cancel button.
You'll be fine, if I break my current phone, my next one will be a Galaxy S4 as well, some people do run Android 12 on it nowadays, the battery is replaceable and the phone is so cheap used that it's not even worth worrying about.