Software for ISP

Hello. I read a lot about creating my own ISP, but everywhere they write either about the initial stage of collecting documents and other paperwork, or they describe in general terms. Examples on Habré here and here
I appeal to those who work / worked in this area - please tell us how your software part works. Interested in absolutely everything.
For example:
What billing, samopisny or purchase? How does he work?
What systems for technical support do you use?
What monitor the network? What are you doing against sniffers?
Etc.

If someone pokes his nose at the link to this topic, I will be very grateful.
Thank you in advance.