Every single one of them was deploying the projects on the internal infrastructure, sometimes a small Kubernetes cluster, some other times just an EC2 instance, most of the time just one or two VMs.
Each employee was given a computer with a VPN client pre-installed, to connect to the internal infra.
The biggest company I've worked with had 10k employees, still not enough to see any impact on performance on the slowest single-threaded service we made.
When deploying, we were simply adding to the internal DNS server an A record "service.internal-domain.tld" pointing to the IP of our server. No load balancer, no reverse proxies, no HTTP/2 in the stack.
It's cheap, it works, it's certainly not something you would do if you were serving a SaaS application to millions of user on the world wide web, but it's definitely enough for internal stuff.
Even Google does not have more than 100k of employees.