Real wood furniture infested with pests - what to do?

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I also don't really see why I should take care of getting rid of the pests. The cabinet is 3 months old, so the seller should take care of it.
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It would possibly have to be proven that the pest was already present when the furniture was delivered. A phone call clarifies whether the seller will do something about it. At least in our circle of acquaintances, the carpenter was very accommodating.

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I am more afraid that the problem will "spread" to surrounding furniture? Is that likely?
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Almost any wood with sufficient moisture and starch content can serve as food! The piece of furniture should be removed from the house immediately or at least wrapped in foil.

No, in the first six months the seller must prove that the defect was not present with him. Only after that does the reversal of the burden of proof begin with the warranty.

Wikipedia helps under "Common woodworm beetle". The danger of spreading to other wooden parts mainly exists in the next generation of these little pests, following the motto "Let's find a nice nest with enough food for our little ones."

I need to look up how long such a generation lasts. But even that doesn't help me much, since I don't know how old the wood is.
I have now arranged with the seller that they will pick up the sideboard as soon as possible. We will then receive a new one step by step, as it is currently not in stock. It could be by the end of February.
I'm considering asking if they would take the item back completely. Maybe I should also get rid of the other two pieces of furniture and buy completely different living room furniture. (?)
Would you somehow attribute this to the quality of the furniture? Or can such an infestation sometimes occur when buying real wood furniture? I'm really quite shocked.

Above all, you don't know if the dealer's entire stock is affected by it...

Such an infestation should not occur, but it can happen occasionally. If the other furniture shows no signs of infestation, they are fine as well. Stick to the manufacturer and line, you didn’t choose them without reason, and assume that you just had bad luck with an isolated case. Don’t let it drive you crazy.

In fact, there are indications on the internet that something like this can happen when buying furniture. I would have assumed that antiques are more likely to be affected, since appropriate precautions should be taken during the manufacturing of furniture.

If something like this goes unnoticed, it can end badly. In case of (very) extreme infestation, you can even hear the critters "smacking" during rest (at night). No joke. But we are probably far from that here ... :)