I won't continue beyond the following statements:

Adjusting trim holds control surfaces in fixed, non-neutral positions.

No, it doesn't. Aerodynamic forces still work on the control surfaces,
and they are still free to move.

The autopilot doesn't use trim to relieve control pressure.

Yes, it does. It just doesn't sense pressure the same way a pilot does.

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.

That statement is a perfect indication of the difference between "what
button do I push?" and "what happens when I push the button?". It may
"cause the same thing to happen", but it causes it in different ways.

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

This is an acceptable but arbitrary definition which would work for
aircraft with trim tabs. This putative neutral position however has no
aviation significance (it doesn't correspond to any useful pilot input
or requirement), and is utterly meaningless for other trim methods, such
as bungee or jackscrew.

And that's all I have to say about that.

Jose
--
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