In particular, serie 36 is for the LTE RAN (radio access network) while series 38 is for the NR (~5G) RAN. But other series are relevant, for the core and services parts among (many) other things.

All the specs are publicly accessible, and also the change requests (CR) submitted for discussion at the standardization meetings, as well as the minutes. From a spec page you can get to its history and the related CRs. Now of course, there are side discussions but the process is still very open in practice. But it's really huge, don't really expect to make sense of it with a little casual looking around. It takes some real time investment and commitment.

Another point to keep in mind: the cellular industry has one of the most demanding certification process for consumer grade devices. And operators often add on top of this. You don't want buggy devices messing up a shared medium, particularly when it's a scarce resource in many places (high density urban area at peak time). If designing a cellular modem is hard, making it go successfully through certification and also work well in the field and pass the tiers 1 operators field trials (drive tests in particularly harsh locations) and interoperability tests is even harder.

A lot of people miss this part. It takes millions in test equipment just to get ready to enter certification, and that's not the main cost. Even if it's painful, it's also important to make sure things work reasonably well in such complex, multi-parties systems. Which is why certification is legally required to operate in many places (e.g.: Europe requires GCF certification). There are open implementation of LTE, but none certified and legally usable in the field because of this.

At the beginning of my corporate career I was working on implementing LTE layer1 (PHY) in internal testing tool. It sometimes took us weeks to understand a single page from the specification - those were fun times ;)