Thank you, I use Electrum.
From all your post and others' so far, it seems that I can recover my wallet with seeds on any other devices.
Backup files and password of backup files are only helpful if I lost seeds, but still have access to backup files & passwords of those files.
Do I understand it correctly.
Yes. To recover your wallet if anything happens you will need EITHER:
a) your backup seed.
b) your wallet file (and if encrypted, your password).
c) another device with the wallet so you can get any of the above and restore in your other devices.
Remember: ALWAYS write down your seed in a safe place. Thats what matters the most.