Notes on android

What is best and easy to download notes program FOSS on android (e.g. which can be downloaded from google play or best f-droid)?
The main feature for me is it that I will have some folders and will be able to back-up notes in easy to use format (e.g. txt) on my computer.

You can use Markor for that. Another suggestion is Joplin if you need built-in encryption.

I really like Markor and use it all the time.

If my phone has encryption, I guess I do not need built in encryption?

No you stil need it without it they can read your notes

Again, this depends on your threat model.

If you assume that an attacker can’t break the encryption of your phone and your phone is always encrypted when the attacker gets it, then your phone’s encryption should be sufficient.

If you assume that an attacker can break the encryption of your phone or can directly steal the file with your notes (e.g., you go to a malicious website that somehow extracts this file, or you install a malicious app that copies your files), then your phone’s encryption won’t help.