[AC Question]

Trying to set up my emails and have finally decided to use gmail and link my old yahoo account to gmail. My question is why I need to sync gmail? Does this sync my yahoo emails to gmail or is it backing up the phone emails to a cloud somewhere? Or it it neither and I have no idea what I'm talking about.

[LeoRex]

'sync' is only describing the process by which the local gmail service automatically updates the inbox. Your local gmail account is just a mirror image of the same account on Google's servers. When it 'syncs', it is keeping your phone's gmail account up to date.

You phone's emails are, indeed, stored on Google's servers.

[Rukbat]

And your Yahoo emails are still kept on Yahoo's email servers, and synced to your phone.

You don't have to link the accounts. You can set up a gmail account and a Yahoo account separately on the GMail app (or any email app).