My admin user password stopped working on mac after connecting it to iCloud using old macbook account

Problem: after using iCloud account my previous admin password stopped to work on new mac book.

Context:

• Mac book PRO 2015 (with replaced battery by mac 2 or 3 years that need replacement again as it is swollen - synced with icloud)
• Mac book PRO m1 2021 (I didn't sign with icloud as I wasn't sure if I want to have data from older mac)

As I move to another country I decided it's time to give away/sell my Mac book pro 2015 but I wanted to sync calendar/etc with MacBook pro 2021.

I signed in on new mac with my apple/icloud and I have notes/calendar etc, but when restarting the computer my old password doesn't work.

Why? what happened here?

I can't compare user account with old mac as I format it and wanted to reinstall system (I didn't finish as computer get crazy hot and fans were working hard on 25% - I didn't want any emergency with battery so I will leave it to some hardware specialist or I will throw it away).

My user name in /Users is "dev" but on login screen it shows my name (I think before it showed "dev" as well on login screen - but as I don't pay attention to it and usually have sleep mode I can't be 100% sure). Second standard user is "testuser".

Is there anyway to see when account was created so I can be sure the account is the same as it was when I bought new mac book? How to restore access to admin account? Thankfully I did partial backup for my personal stuff, but work stuff, etc. will be gone if I won't be able to restore the access to my admin user account.

Is there any other way to get restored admin account?

I'm owner of mac books, I bought it on my business activity so it's easy to verify I'm the owner of it.