Which grants are needed to access a table?
There is no simple list of necessary and sufficient grants, because there are multiple paths that can be used.  CONNECT on the database and USAGE on a schema are often done through "PUBLIC" rather than to individual roles, for example.

Newly created databases will have CONNECT privilege for PUBLIC by default, so unless you have gone out of your way to remove it no explicit grant is needed in that case.  When you create a database, it comes with a schema named "public" which also has USAGE granted to PUBLIC, but schemas you create manually do not come with that privilege.