MySQL still using one CPU even though extra CPU added
Short answer:  MySQL never uses more than one CPU per connection.  It never has, and there is very little hope on the horizon.
Exceptions:

There are some background threads that many use a small amount of CPU.

There are a limited number of cases in 8.0 of making use of multiple CPUs.


I would argue that it is not worth much effort to make use of multiple CPUs:

The orchestration of splitting up a query and recombining is a lot of overhead and complexity.
Some things virtually prevent using multiple CPUs.  Think ORDER BY.
Many queries are I/O-bound.  Extra cpus don't help in that case.
The typical &quot;busy&quot; system has many connections doing work.  In that case, many CPUs may actually be used.