Proxmox is a hypervisor, which is an OS that is built to run Virtual Machines (proxmox also runs containers). It is open source and can be installed for free, just like any other linux distribution, the same way Windows is installed. There are tons of tutorials out there on how to use it.
From there, you could setup some popular containers, including nextcloud, or even install full OS's in virtual machines to install software manually on them. It is a great first step, especially if you have limited access to hardware.