You can use these queries:
select * from all_tab_privs;
select * from dba_sys_privs;
select * from dba_role_privs;
Each of these tables have a grantee column, you can filter on that in the where criteria:
where grantee = 'A'
To query privileges on objects (e.g. tables) in other schema I propose first of all all_tab_privs, it also has a table_schema column.
If you are logged in with the same user whose privileges you want to query, you can use user_tab_privs, user_sys_privs, user_role_privs. They can be queried by a normal non-dba user.