I am a startup among the other things. So far I feed myself and was for the last 20 years ( startup of course is not that old ). I host my own servers and also rent dedicated servers elsewhere. I have no need for GSuite and Zoom. I talk using Skype which is free. I use vertical scalability and my servers are C++ so this infrastructure serves thousands requests per second without breaking sweat from a single server. In the end my current app serves tens of thousands of clients with very little monthly costs.

Microsoft and other do own their infrastructure but if they're not renting it to clients it wastes money as it has ongoing costs so not, them using their own infra is anything but free.