SNMP has its own protocol, both for data inquiry and security.
In POSIX unix there are several files to control all of this. Net-SNMP, although I've never used it, has ways to implement all of this stuff.
By default SNMP uses UDP on port 161. You can create a 'community', with clearly defined access levels.
Instead of this forum, read the Orielly Book
Essential SNMP - O'Reilly Media
SNMP protocol is a product of DARPA (US dept of defense). This means it verges on idiotic as it was conceived by bureaucrats. And a meaningful undertstanding of what you are asking is way beyond the scope of a forum.
Example: the answer to each of your questions varies by version of SNMP - 1, 2, or 3.
Wait til you read about MIB files. There is a whole book just on the mib2c utility.
My Solaris conf files for snmp (and there can be more than one) are huge, and are maintained by special scripts.
There is no simple shortcut to all of this stuff. Sorry. Without the right background it is easy to mess things up. IMO.