OK, I think I might know more of your problems.
Did you import all your contact files to your Google account? When you go online, open or sign into your Google account. Easier on a PC, then find the contacts section, and explore the menu. At top are typically phone contacts, down near bottom are "other contacts" which are all addresses of email sent. Now, if you've not imported, and merged all your contacts, then it like starting over from scratch. But, there is an import, export function in menu. What I did originally was imported all my phone number contacts to my Google account, and all my email built up on their own. Which I have nearly 400 addresses. When you start typing an address, you'll get a list of most frequent, matching the spelling first, then scroll to the one you won't, tap to finish auto load. This is how gmail works on Android. Now, you could use gmail to manage as many addresses as you want, or use separate apps, irregardless, they all will pull from the main contacts lists. That's why in your synced accounts, main phone settings app, Google has all those toggles for everything they back up, namely contacts for one.
Here are a couple links to help get you pointed in the right direction. First, shortcut to account contacts google is managing for you.(you'll sign into Google account like normal to access)
https://contacts.google.com
Secondly, a help article on gmail. This should help you understand how Android email clients operate. Also, there are settings you'll want to review, so explore this after you open the main Google account and select gmail. (You can navigate between sections once signed in.)
https://support.google.com/mail/ans...4369203091485-3624539253&hl=en&rd=1#shortcuts
Note: (if you are accessing on mobile, scroll to bottom of menu, tap "use old style" which is probably better on bigger screen)
And if you want to start on the main page: Google account, that's fine too,
https://accounts.google.com/signin/...flowName=GlifWebSignIn&flowEntry=ServiceLogin
One last short article on gmail might also be helpful.
https://www.cnet.com/how-to/how-to-disable-gmails-automatic-contact-creation/
To give you an idea to find if it's enabled it not. Which is why you want I suspect, to have it turned on, not off. But, if you didn't import all contacts from other email clients, then that's what needs done.