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.
You need to identify the toilet. There is often a brand logo under the toilet seat, between the bolt holes.
That said, it appears to be a Duravit “darling new” wall toilet. There are two or three models within the range, each gets its own seat/cover. Measure the length of the top of the bowl from wall to front edge and it will correspond with the spec online at www.duravit.us. It looks like the elongated version which probably uses toilet seat 0063390000, but you should check.
Good luck!
Best Answer
Use a Dremel with a cut off grinder wheel. Works fast and precise. Buy new screws. No holding of screws required. There are thousands other purposes for the Dremel tool you will discover. And it's fun.