Backstage is built on a modular, plug-in based architecture that allows engineers to layer-up their developer portal to meet their own needs.
Frankly, this is just incorrect. Just read the configuration guide for "installing" a plugin to see what I mean. You have to modify the source code of your own deployment in several places. That's not a plugin based architecture.
https://backstage.io/docs/getting-started/configure-app-with...
Additionally, check out what it takes to stand up a standalone backstage server. You used to have to clone their repo if I remember right, but now they give you a starter template via npx and leave it up to you to put all of the pieces together. They don't even ship a docker image to get you started.
https://backstage.io/docs/getting-started/
Frankly, the architecture of Backstage is ill-conceived and a hassle to operate. If you have the budget their are vendors that will deal with all of this for you. I think the concept of Backstage is fine, but the execution leaves a lot to be desired.