Creating one user access account for all the multiple sites

I am new to WordPress and ill just state the situation.

We have multiple websites created independently and I need to create a single account for the subscribers for all the websites like google.

I am asking for advice and help on how to build this and I read multiple forums that WPMU is capable of doing this but it has limits like for example it throws the WordPress wp-admin as a login page maybe I need a custom login and dashboard for users/subscribers and monitor each of there activities like example we have an e-commerce website that needs an account before purchasing same to the blog websites that needs an account first to access contents and an e-learning website accessible with one account just like google but on WordPress platform

Can you guide me on how to do this like tools/knowledge?

Thanks in Advance.