Grants and privileges issue

Hi, It's postgres 15.4.

We want to give required privilege to certain users or roles and ensure to
not to provide any elevated privilege. I have below questions,

1)I am seeing in many places, we have "usage on schema" privilege given.
along with "grant select on <object_name> to <role1>" for the objects of
the schema (something as below). So I wanted to understand, what exact
privilege "grant usage on schema <schema1> to <role1>" will provide which
the "select on" privilege won't?

grant usage on schema <schema1> to <role1>;
grant select on schema1.tab1 to <role1>;

2)Additionally , when we are trying to give select privilege on "cron" and
"partman" schema to a role (something as below) , so that anybody logging
through that role would be able to see/fetch the data from the tables
inside cron and partman schema. its giving output '*no privileges were
granted for cron/partman/part_config*' message. And during accessing that
object from the cron/partman schema through that role, it errors out with
an access denied message. So I wanted to understand the cause of this and
how we should fix it , such that anybody logging in through that role can
see/fetch the data from the cron and partman schema tables.

grant select on cron.job to <role1>;
grant select on cron.job_run_details to <role1>;
grant select on partman.part_config to <role1>;

Regards
Sud