>>Can USER A login to SITE B?
Yes. WordPress multisite has the single users database table. User logins to the network, not to the site of the network. Then WordPress looks, what role(s) user have at the site, which he opens. Every site has own permissions. So user A will not have any permissions at the site B and may get probably ‘You do not have enough permissions’ error message when try to open wp-admin at the site B.