VRF understanding

I’m having trouble understanding how VRF works. I have 25 sites and I want to start segmenting each site because they have networks with building automation equipment etc. Basically stuff that shouldn’t be accessible through interVLAN communication. My question is how does VRF work when I segment it at the site and up to the backbone switch? I plan to keep segmenting up to the firewall, but I want to do it slowly so nothing breaks. How can I slowly do this? I’m just not understanding how to get the traffic out of VRF at the backbone.