The advice I've always heard is disconnect network but leave powered for forensics/recovery. Some ransomware store the decryption key soley in memory, so it is lost upon power loss
That actually makes sense. We had a ransomware attack once. We also disconnected the device but I cant remember if we powered it off. At the time it stopped encrypting due to that since our network drives were not reachable anymore.
Is there actually a way to spread the encryption process to a server?
I vaguely remember the advice actually being to leave it running but disconnect it from the internet. Although maybe hard disconnect the backups if you can.
There could, in theory, be a malicious machine on the internal network that was previously infected, which is now acting as command and control. So if you didn't know which one it was...
Depends. If you're at home with a single endpoint, maybe.
But in cases like the image there's a lot of internal traffic and you'd want to stop the malware spreading internally. There might not even be internet connection at all.
Most serious infections are able to work within isolated internal network. You can stop data breaches by cutting external traffic but if you have ransomware you might want to cut internal connections too.
You might be able to stop the ransomware from triggering on some devices. That of course depends on the type of ransomware and whether it's triggered based on time, external command or something else.