Neighbor is building a retaining wall on my property. What should I do?

This only applies in very rare exceptional cases. Boundary disputes are a private matter. The surveyor only comes into play when the parties do not agree on the location of the boundary points. And someone has to pay him then.

Yes, that's correct, but don't set your hopes too high. The authority has nothing to do with the boundary violation, and I have already commented on the options regarding violations of the development plan in #8.

At the moment it is very important to establish the facts, collect evidence for timely objection, and document it in a legally sound manner. If the WhatsApp chat is lost due to a new phone, that could be annoying in a possible upcoming trial.

I warn against WhatsApp. Partly not recognized as evidence. Go to a lawyer.

That only applies in very few exceptional cases. Border disputes are a private matter. The surveyor only comes into play when the parties do not agree on the location of the boundary points. And someone has to pay him then.

No, experienced it myself! My shell builder dug out the boundary stones and didn’t put them back in! 2-3 weeks later, a letter came from the building authority. A month later, the surveyor was on the property and the boundaries were remeasured, additionally the position of my L-stones on the boundary was checked. All affected neighbors had to be present.
Fortunately, we did not build over it. It was 4 cm too little which now hangs in the air.

There were no disputes, no exchange, no communication. Someone, no matter who or some bored grandma, grandpa, cat, dog, etc. saw and reported it. Which is also okay.

That’s why I say, dig out and report. Yes, it has to be paid, but who pays if you don’t know who it was?!

In our case, it was clear.

My shell builder dug out the boundary stones

Then the culprit was known.

and did not re-bury them!

That would be even better.

2-3 weeks later a letter came from the building authority.

Building authority is a collective term for all construction-related offices of a municipality.
From which office exactly?
With what content?

and the boundaries were re-surveyed

What exactly was done? Were the boundary points remarked?

All affected neighbors had to be present.

At the surveying appointment? Which federal state?

There were no disputes, no exchange, no communication.

Are you sure that the shell builder informed you about everything in this regard?

That’s why I say, dig out and report.

Then don’t get caught (VermKatG NRW, other federal states have similar regulations):
§ 27 Administrative offenses
(1) A person acts unlawfully who
...
4. intentionally or negligently attaches, removes, or alters boundary markers contrary to § 20 paragraph 5,
...
(2) The administrative offense can be punished with a fine of up to fifty thousand EURO in the cases of paragraph 1 nos. 1, 2 and 6, and in other cases with a fine of up to three thousand EURO.

For us it was clear-cut.

Who paid then?

This is not a tip from me, but if boundary stones are dug up, the district office has to send a surveyor.

Is it really that easy to just dig up the boundary stones? Then anyone could arbitrarily enlarge their own property. Would it make sense to have this checked or does the surveyor do this anyway in connection with work for a new building? On our "post-war property" here, only the two boundary stones in the sidewalk are firmly attached. One is completely missing and the other is just a large, angular stone about 1 cm in the ground, without any further marking. No one has ever cared about that before.

that has to be re-measured. You cannot rely on the boundary stones (especially the old ones). Ours were more than a meter off...

Is it that easy to dig out the boundary stones?

Sorry. It's as easy as driving 100 km/h in a 30 zone or hitting someone or otherwise causing harm. I would like to add the OWI here with Par. 274 StGB.