Password manager

Basically the same as in Windows: Keepass with manual sync between devices(using Syncthing for example) or Bitwarden (Vaultvarden if like you like to selfhost and don’t have enterprise account).

image and PDF viewers

I’d use a desktop environment defaults, but https://wiki.archlinux.org/title/List_of_applications

grep’s powerful

Awk and sed are great too. Sed will also turn 50 this year.

how does regex

It’s magic. You can(and should) test your regex here https://regex101.com/