Contact details on old MS account not available with Authenticator

Maybe this will help dear sir. There is a way to recover your old password. Try here.

Hope this helps.

You can't really merge the accounts, but you could manage the subscription of the accounts, and possibly reach out to support to cancel the subscription on the one account for a prorated (again MAYBE) amount and subscribe on the other account to consolidate
the services, just not the accounts.

Which account is it you don't have access to? I'd assume it's the Xbox Live/console account? If so, you'd at least have the email address which is good, but then you'd have to go through

Account Recover here. That might involve contacting Microsoft Support to verify your identity at some point due to the two factor authentication.

Let us know if you have any specific questions or problems with the Account Recovery process.

You can not merge Microsoft/Xbox accounts.

MS will never ask for account info. The will never reach out unless by request. They have an authentication procedure they follow very strictly before they can even talk in any detail about your account.

I suggest contacting MS Support and also check your MS account for subscription details.