It’s actually pretty well documented and the rest of the details can be surmised by looking under the hood a bit. Bubble is using NodeJS on the server and the data store is a postgres backend with a proprietary storage format (also known) as they are able to give direct database access to dedicated server customers.
Some really interesting articles are here if you really want to get in the weeds:
Basically the framework is almost entirely based upon Javascript and a custom JSON parsing engine. It’s quite a fascinating and innovative piece of engineering (imo).
Further reading here that gives insight into what is under the hood: Bubble donates $20k to open-source projects
If you get through all of that reading and have a specific question feel free to post again