Learning about Transcoding/ Bitrate

[DeanoVIP]

I'm going to embarrass myself but my knowledge of encoding, transcoding and bit rates and audio is terrible at best.

I've got a bunch of formats h264, h265, some say HEVC or 10 bit.  I see sometimes people write X265 not h265?

My understanding is 264 and 265 is newer offers better compression and still good quality? 

No idea where HEVC comes into play, if anyone can shed some light.  Most audio is AAC.

Where does bit rate come into play, if it's 5mbps I assume that's what it's sending to stream the media file?

And how does Jellyfin decide when to transcode or just just stream a file as is directly.