Yes, it does have a microSD card slot. The website isn't the most clear website ever, but if you click on "Specifications" and then "See all specifications" everything will be shown, including a MicroSD card slot
I have a 512GB microSD card in my current phone that is about half full. I don't have any interest in paying a subscription to store my stuff on someone else's server ("the cloud"), when the SD card only cost me a couple of months of what a subscription would. I periodically back up the SD card at home.
As to what's on it? The usual stuff like pictures and videos, but also game ROMs, call recordings, shows/movies, ISOs, utilities (I can make my phone appear to be a flash drive, bootable even). I also backup my texts and other records using a third party app. When I do have to swap phones, transferring the SD card over and then restoring stuff like settings, messages, records, etc is way faster than any of the OEM transfer tools.
Music mostly. I have over a terabyte of FLAC files stored on my home NAS and whilst I do have a Navidrome instance setup to stream it all, it's not as reliable as just playing locally.
In theory, but phones do soooo much with the NAND that I think it varies quite largely.
I've struggled to get more than 3-5 years out of my devices, and its seemed to be the NAND causing the failure in my limited experience
My galaxy S4 failed after two years and would go straight into firmware flashing mode when connected to a computer. Leading up to this, it was reallllyyyy slow. I eventually narrowed this down to the internal storage, and moved my apps to the SD card where things sped up again. It would also frequently reboot.
My Galaxy S5 (RIP 🤧) failed after a good 6 year run, now it goes straight to recovery with MMC_READ failed sadly
On the opposite end of things I've got an old Android 4 tablet from 2013 that still works perfectly fine, although I don't really have any reason to use it, it's kinda just existing as a time capsule.
Thanks for sharing your experience :) Glad I never had such issues with my devices, but I guess it heavily depends on the specific device and the usecases.
My current Op 7 Pro has also been running without a problem. But in case it happens I know what might be the cause