Jose writes:

Trim =is= used to relieve control pressure (*).

That is only one of multiple possible uses for trim.

Adjusting trim holds control surfaces in fixed, non-neutral positions.
Relieving control pressure is not the only reason for doing that. The
autopilot doesn't use trim to relieve control pressure.

You seem to be asking (without successfully communicating the
distinction) "what, in detail, happens when trim is adjusted?", the
answer to which depends on the individual setup.

Trim pretty much causes the same thing to happen fundamentally on all
aircraft.

You also got hung up
on a "neutral position", which either doesn't exist, or can be defined
arbitrarily (just like the w/b datum).

A neutral position is easy enough to understand. Neutral trim produces the
same effect as the complete absence of trim tabs.

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