Questions on syncing contacts on iPhone.
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CallMeAl
macOS_Skyline
Unfortunately, if you want to combine contacts from different accounts, it’s a bit of a mess. The easiest way to do it is to create a blank contact in the destination account (in your case iCloud) and link it to the contact from the other account with the “link contacts” option near the bottom of the contact menu when you are in the edit screen. Sorry if this explanation isn’t particularly helpful 😅
rant · world
you can go to settings and accounts for Mail. Click accounts. Select ayy account and click the Contacts Switch to disable it. Contacts won't be saved to that aaccount. Be sure to remove any contacts from that account
CyberGeek
There should be an option inside of the actual Contacts app that will allow you to select a default account where new contacts will be stored. And if you only want a single Contacts account option, then you need to go into all of the other accounts you have connected on your phone, and turn off their contact option.
Jasmin
Thank you.
Blue Blaze
You could get back your yahoo account if you have a recovery email and if you know your recovery email this thing happened to me one time but not with the contacts it happened to one of my social media and somebody had my old phone number and then lucky thing they gave me the code for it, which I was thankful but they was cool about it. They understand my situation. 
macOS_Skyline
Yes, you can absolutely disable contacts on an account by account basis. If you go into “Settings”, scroll all the way down to “Apps” and  select either “contacts” or “mail”, the very first option should be “Contacts Accounts” or “Mail Accounts”. If you click into that option, it should show you each of your accounts in a list, and from there you can open any of the accounts and uncheck “Contacts” if you never want your contacts app to try synchronizing them to that account. Additionally, if you would just like to view which contacts are a part of which account, you can go into the “Phone” app, go to the “contacts” tab and press the button in the top left corner labeled “Lists”. The first option in that list, “all contacts”, that’s exactly what it sounds like, all of your accounts together. But beneath that you can drill into each individual account, for example, iCloud, Gmail, etc..