Avast one auto update

I’ve heard avast one automatically updates virus definitions. When avast one was opened it said Important update for Avast one Please restart your pc
To finish installing this update. Is this just a general update?

Whilst I don’t use Avast One, this sounds more like a Program Update (which generally will require a system restart to take effect).
I believe the default setting for Avast One is to Automatically download and update Program version updates.

For the most part, virus signatures don’t require a system restart to apply the update.

So most likely virus definitions are being installed automatically but a software update is just manual

Depends on your program settings. I dont use avast but i think default is auto for evrything
Some program updates may need a restart just like Windows updates

Lots of info here if you scroll down to search https://support.avast.com/

https://support.avast.com/en-ww/articleSearch/?searched=Update