It’s a tradition at this point. If you post an infographic about unix/linux system folders, you’re obliged to avoid all modern sources. Preferably, you would use material that is at least 20 years old.
/mnt is reasonable and normal. I have used /mnt, /data, /media for various hardware and software mounted storage. It really doesn’t matter unless you’re dealing with some specific software or organization with esoteric requirements.
/mnt or /media usually. I use /mnt for permanent filesystems and /media for removable ones but there are no hard rules. My home folder is a separate filesystem from my rootfs, just depends on how you want things setup.
Temporary files can be created by user programs. On my machines, I made /tmp an in memory file system and also disallow execution or setuid/gid in this directory as much malware tries to abuse it in this manner.