The VideoLAN Forums

I would appreciate it if you help to find out where is HEVC processing in the source code.

It depends what you mean by that

I'm interested in how the library handles HEVC decoding if in the client's operating system priority (by default) may be a graphics adapter that does not support HEVC profiles?

VLC does not have that capability. That's done in underlying libraries.

Please, tell me more details about these libraries.
What are their names? What API of libraries should I pay attention?