There are two (at least) "beasts".  In a formal motorcade, they always travel together.  Both have the DC "800-002" tags.  You can see the second one behind the limo in this news photo:
They have been using the armored Suburbans more lately for the less-formal motorcades.  The Presidential Suburbans also carry the 800-002 tags.
Yes, you can do that in DC.
The limos are supposed to have adjustable tinted windows, but I think it's the same technology that is available from Horton, PL Custom, and MedTec for ambulances, which quite a few rigs around here use.
Biden flew into the airport where I work, but he had a Suburban.  I don't know what he rode in yesterday.
Again, nothing secret here, just stuff anyone can see from the street or on TV.