i have multiple website with multidomain but i want the same database

i have multiple website made with diffrent themes and plugins on a single domain like
https://abcxyz.com/site1
https://abcxyz.com/site2
https://abcxyz.com/site3
https://abcxyz.com/site4
with their own separate database just like a normal subdomain

Note (these website are ready to 100% completed)

also i have almost 15 domains like
https://mgf.com/
https://klm.com/
https://efg.com/
……

what i want

1) First i want to have the same login for all these subdomain for example if u login into 1 website u login to all the sub-domain something like multisite.
https://abcxyz.com/site1
https://abcxyz.com/site2
https://abcxyz.com/site3
https://abcxyz.com/site4
————————
2) second i want all the my 15 domain should b the mirror of this domain and subdomain somethig like domain mapping.

https://abcxyz.com/site1
https://abcxyz.com/site2
https://abcxyz.com/site3
https://abcxyz.com/site4

how this is possible ??

regards,
Ameer hamza
any help will b greatly appreciated ??