Plumbing – What are the kitchen sink’s drain pipes made of, and how should I seal connections

The issues with the compression fitting have been addressed.

The brass flange fitting, the rubber gasket, and the threaded pipe is the "toilet spud". Replacements are available if the rubber is perished or the metal parts somehow damaged.

The flange nut looks pretty well bonded to the threaded portion. Soak well with WD-40 and let it soak in. There are splines inside the spud on which one would apply an internal spud wrench to keep the threaded pipe part from turning. You may find some other chunk of metal that will wedge against the splines, which in turn can be held by a conventional wrench. Holding the spud in place, remove the flange nut with a suitable tool.

Once the flange nut is removed, push the threaded pipe part inwards, but ensure it will not fall into the toilet. The part inside the toilet is conical shaped. Pushing it inwards will give enough free play that the rubber gasket can be pulled out. Once the rubber is out, there will be enough clearance in the toilet hole that the threaded pipe part can now be removed.

Examine the parts and evaluate if replacement is necessary. If the rubber is not too far gone, it may be possible to clean it, treat it with some sort of rubber treatment, and reinstall. If the rubber is cracked, hardened, rotted, or brittle, it will need to be replaced. The nut should be tightened firmly enough to ensure the rubber seals well against the toilet, but not so tight the rubber gets pushed out of position or the toilet gets cracked or broken.

I would call it a slip x spigot brass drain street ell. Can't say I've seen one until now. More common is the double slip brass drain ell. Hard to say from a photo, but looks like a short horizontal straight piece could be inserted if you had to use a double slip ell. Perhaps the trap U could be rotated away a bit to buy a little extra distance for this.