One user hack = undetected?

It's my understanding that hacks have to be updated every time CSGO and VAC update, to avoid the hack being detected.

Is this because it's possible pattern's in the hack's code were uploaded to VAC and are recognized? If not, why so?

And if I made my own, personal, custom triggerbot, and didn't release it on any sites, what are the chances of it getting detected? Would I still need to update it every time CSGO updated?

Thanks