The Bluetooth specifications ensure interoperability between products. The majority of practitioners usually don't need to reference the very detailed specifications because they are using software and hardware that already adheres to the specifications.
Bluetooth has a core specification that has undergone versions 4.0, 4.1, 4.2, 5.0 and 5.1. The versions are backward compatible in that devices supporting, for example 5.1, can work with devices running lower versions, obviously, as long as the newer features aren't used.
There are also separate specifications for GATT and Mesh that coexist on top of the core specifications. The GATT profiles provide specifications for common usecases. For example, the Heart rate profile (pdf) allows hear rate monotors to share the same Bluetooth heart rate API.
What impact does the deprecation and withdrawal of Bluetooth Core specification v4.0 and v4.1 have on Nordic current and future products?
What You Need to Know About Bluetooth SIG Deprecation of BT v4.1 and Older Specs
CC2541: consequences of BT 4.0 depreciation
Bluetooth Core Specification v5.1 - What You Should Know
NIST Guide to Bluetooth Security (pdf)
Bluetooth® 5, Refined for the IoT
Understanding Bluetooth LE Advertising
Understanding Bluetooth LE Reliability