Answer: yes, some code. But other code I and my company like to keep private.

It's common for companies to have something like self-hosted GitHub Enterprise or self-hosted GitLab hidden behind the company's VPN.

But where is the box where it's hosted? Is it in-house?

There are alternatives out there for self-hosted git. I have a Gitea instance running on a mini PC at home for my own projects.

Do you have backups of that as well? If something were to happen to your mini pc would you lose your code?

Great question, yeah I do. Right now it backs up to a separate NAS on my home network. Every once in a while I'll copy the most important directories onto a microSD card backup, but its usually going to be at least a few weeks out of date.

Own servers.

Do they manage their own servers? I wonder what proportion of companies would have in house servers managed by themselves.

They are colocated in a data center and you need physical keys to access the rack.

Internally hosted gitlab instances are a thing.

They are, but frequently the boxes where they are hosted are in AWS or similar. Or do frequently companies have actual in house servers for this purpose?

Not in house, but in a "segmented" part of the cloud that comes with service level agreements and access control and restrictions on which countries the data can be hosted in and compliance procedures etc. etc.

An extreme example of this would be the AWS GovCloud for government/military applications.