In order to understand user login policies, you need to understand the basic technical facts: there are databases, and users have user accounts in those databases.

at this time there are two actual physical databases:

adhoc :

network :
consig pen

there are also development copies:

sandbox tbo
sandbox network

A user can only log into a site if that user exists in the database. Thus a tbo user cannot log into yaamny, because the database is not the same.

A user is not ALLOWED to login to another site on the same database. ie. a yaamny user is not allowed to login to itsmypark. but this rule can be changed as you like. There is no technical barrier.

The dev sites share the same database as the live sites, and thus the same users. The sandbox sites, if it has been recently updated (see backup ) will have all the users that the live sites do.

Cookies are only readable within the domain they were written on, so there is no way technically to recognize that somebody has been on another of your sites. You could do some kind of redirect or hidden image that writes to the other site.

at this time:

all the login code is in login/Access.php and login2/Access.php and you can change or set your policies here.

is for username/password logins.
on your system users don't usernames and passwords, so they can't log in here.
but they could if/once they get u/p

editors and admins can log in here.
admins can log in to any site that is in that database.
editors may only log in to the site they were created on.
when you have groups and permissions in the way you want them, then you can
change the policy as you like.

is for fname/lname/email logins.
only users may login here