In the standard install, you log into the same place, regardless of the site- and can access different sites via a dropdown in the main navigation. That said, if you wanted to access them from different urls- say you have a different domain for each site and want the cp accessed from the domain associated with each given site- you can use a copy of admin.php and basically follow the mask cp access docs to have a cp in each domain.
As to sharing variables/tags? The list is long- the best way to get a feel is read over the docs, particularly the managing sites section- which lays out the various options- and the MSM variables and parameters.
Make sense? And I’m going to shift this over to ‘General’ as it’s not really an installation issue per se.