How to log in from a non-domain PC to a server in a domain on a share, and where does the “Guest” come into play?

Question

Active Directory

The essence of the problem:
There is a server in the domain and there is a production socket that knows almost nothing about domains and knows only NetBios names and corresponding balls.
You need to go from the panel to the server, to the file ball.
Actually everything turns out, only at the active uchetka of the guest.
It happens like this:
I enter the address \\server\shara
, a login-password request comes out,
I enter the administrator's domain account - it lets me. The connection logs show the following:
Security ID: DOMAIN\server_admin
Target account:
Security ID: LocalServerName\Guest
Random account is not allowed.
How does this mechanism work?