Hey there @BenTheAlchemist! Welcome to the community! The way they are keeping specific versions is building the engine themselves! This is the only way to make sure you don’t get any hotfixes as well as full control over the installation. To get started, you need to link your Github account to get access to the source code:
https://www.unrealengine.com/en-US/ue-on-github
Then you can follow the instructions here. For specific versions select “Releases” on the right. (this link will 404 error until you are in the organization):