what is mysql database and sys database is about

I read this:
https://dev.mysql.com/doc/refman/8.0/en/sys-schema.html
it seems we can’t restore mysql sys DB from percona xtradB cluster 5.7 to 8.0.19, right ?
how about “mysql” Db, it seems installed by a fresh installation?

Yes you can. You can absolutely restore a 5.7 backup, then install 8.0.19 and start MySQL. The databases will automatically get upgraded to 8.0.19. Downgrade back to 5.7 would be impossible.

"You can absolutely restore a 5.7 backup, then install 8.0.19 and start MySQL."

idea is, if I do not backup the mysql and sys dB from PXC 5.7 to 8.0.19, will it be ok ? what is inside by the way ? mysql should store username and password and grants, right? how about sys? 

You should backup mysql database from 5.7 so that all of your users are recreated as well. You can ignore ‘sys’; it has NOTHING to do with running MySQL. You can 100% delete sys and everything will work normally. The ‘sys’ schema is just a collection of VIEWs for use with performance_schema.

please help on this too:

https://forums.percona.com/discussion/56041/transfer-a-db-size-6gb#latest

"You should backup mysql database from 5.7 so that all of your users are recreated as well"
so if I just backup mysql, should I say I don’t even need this:
pt-show-grants -uroot -pmypassword >mysqlusegrants.sql 
to backup username and passwrod and grants?

Please do not link to other posts asking for more free assistance.

If you include the mysql database as part of your regular backup, then no, you do not need to use pt-show-grants. All username/passwords/grant information is stored in mysql database. You should back this database up as part of your regular backup policy.