You'd be surprised at how fast you can create something with an SQL database, a well used backend framework (eg. Rails) and a well used frontend framework.

The things big tech companies do are usually to avoid scaling problems. In particular scaling on the engineering side. The above can scale to billions on the user side if needed but 10k engineers constantly updating and messing with a traditional SQL schema doesn't work.

You'd be crazy to try to replicate something like the big tech companies database storage systems when you're just starting out. You don't have 10k engineers. You may think "but i want to scale to a billion users". Well you still don't have those problems. The scaling problems big tech companies have are scaling to 10k engineers.