In answer to your question, depends on the manufacturer. Some are glued and some are spin welded. Some plumbing is installed through a tight fitting grommet but I don't believe I've seen the toilet flange with just a rubber grommet.
Since it's a straight drop to the tank, a pressurized water jet should knock it loose and you can get a right angled nozzle. May take soaking and multiple cleanings.
Like some of the previous posts, sounds like the liquid is leaving the tank without the solids. This allows them to dry, stick, and build up.
Once you get it broken loose and cleaned, keep liquid in the tank at all times. I have a long drop from the tank to the valve so I put about 4 gallons in after draining and add chemicals. Only open the valve to drain the tank and if your tank has little in it, add water and then drain. That should keep the solids flushed out.
Some on here say they pour a lot of ice down the toilet and then drive around to clean the tank. Not sure that works but I would think that the residue in the tank would need to be softened up or the ice would just slide around and over it.