Earlier quoted context omitted.
There is a maximum bandwidth at which data can arrive. Simply make sure you can always process it faster than the next packet can arrive, or implement proper mitigation in cases where you cannot. It's called programming under soft real-time constraints.
HTTP/1 between a client-server pair incurs per-request overhead which is not present in HTTP/2. You can do more RPS with less CPU if you use HTTP/2.
You receive an Ethernet frame, you send an Ethernet frame.
The concept of a TCP session is purely virtual, it's just a 16-bit integer in the header grouping the packets together.