Separate three layers
The business API describes methods and objects. MTProto defines secure sessions and messages. Transport carries bytes. Establishing TCP is not user authorization, and obtaining an API ID does not create an authorization key.
Start with the current specification
MTProto 1.0 is explicitly deprecated and belongs to historical reading, not a new implementation baseline. Check current protocol documentation, protocol schema and business schema separately.
Machine-readable schemas and method directories help generate code and locate signatures, but do not replace permission, error or state semantics.
Suggested reading order
Read the overview, key establishment, message structure, service messages and transport before diving into TL encoding. For a messaging product, consider TDLib instead of implementing the protocol yourself.
Our API reference retains source versions and official links. Compare changes before upgrading; do not combine schemas from incompatible snapshots.