I believe what you have there are galls (leaf galls )
an irritation by insects or something causing the leaf to react.

Yes, thank you,looks to be mite galls. They're usually host specific and reported to not be of danger to the host so no need to treat the plant.

What is this species of wasp, several of them like to go inside the substrate for my Sempervivum succulent plants.
They either enter the holes on the side of the containers or enter from the top of the container. The holes are 3.5mm/0.138inches (I used a 3.5mm drill bit), so that gives some good reference for their size.
Most of the containers contain some sort of organic soil and a lot perlite, so there is a bit of room I guess where they can maneuver, especially around the inside edge of the containers.
The total length of the wasp seems to be 10mm / 0.4inches

What are they doing there?

Looks like they are looking for a good nesting space , the holes look just made for them
as for what type of wasp I can not say because I am sure we have different wasps in my area .
It looks a whole lot like a small parasitic wasp. They lay eggs on various caterpillars here. Like horn-worms that like my tomatoes
If they are not bothering you I would just let them be
If you want to discourage them wasps seem to dislike peppermint
I use a spray to keep the paper wasps from my doorway.

I have seen around 3-4 individuals flying around at the same time.

Other times I have seen other black wasps with a very long ovipositor, but those looked a bit different with slightly longer bodies, I assumed those are parasitic wasps, I can't recall those trying to actually get inside the substrate like these ones.

The things can be lay larvae in Sempervivum are some hoverflies that lay inside the leaves and the vine weevil that lays inside the root system. But I have not seen the signs of those being present.

While clearing some light dirt I realized one of the wasps was dead in that dirt so I took some photos of it.

50 Eurocent coin is the same exact size as a Quarter dollar coin.

It could be something like what we call a sawfly. It reminds me of a "rose sawfly". Although called a sawfly, they are actually in the wasp family.

We have that here too, Arge ochropusm, rose sawfly, I have seen such larva on roses two different times, they have a caterpillar-like larva.
But it's not like that one because those wasps have yellow-orange abdomens.
Some of the Pteromalus wasp species will actually be parasitic towards those rose sawflies, in theory.