Basic Questions about PostgreSQL.

-In PostgreSQL, what is the difference between Roles, Groups, and Policies exactly? What are they each, how do they relate to one another, and which one(s) can be assigned to another?

-Is there a simple, immediate, straightforward way to temporarily disable PostgreSQL database
use of all other users, except development users, or in fact just one user (one developer, yourself)
so that maintenance on the data or the data model may be performed?

The way things are, after you revoke connection priviledges for all the other general users,
cancel all the other connections (except for yourself, or developer user access),
and then perform required maintenance on the database. Thereafter, connection priviledges
have to be restored to all the other users, so that they can choose to connect again.

All of this strikes me as being very elaborate. Is there a much more simple
way to disable all and subsequent connections to a database, temporarily, and
then allowing them all again, within the PostgreSQL DBMS?

Is there a public Extension, for example, that installs and works well, which can be used to do this?
Is there a simple, one hit way to turn a set of user connections all off, and then all on
(regarding their permissions and connections, I presume), again?