Question: Standard rate turns, constant rate turns, and airspeed

Working on getting my PPL and I'm trying to figure out the difference
between standard rate turns versus constant rate turns and their
relationship to speed.

IIRC, a standard rate turn is 3 degrees per second and turn you 180 degrees
in 1 minute. The rule of thumb for the bank angle to achieve this is to
divide the airspeed by 10 and add half the resulting answer - i.e., standard
rate at 100kts is about 15 degrees of bank... (So, if that's a standard
rate turn, what's a constant rate one? Something other than 3 degrees a
second?)

But there's an indicator of rate of turn on the turn coordinator that
indicates the standard rate turn. The mark doesn't move. The turn
indicator is a gyro instrument just like the attitude indicator which also
indicates bank. Question: How can the TI always hit the same mark for a
standard rate turn independant of airspeed if, in fact, I'm turning at
different bank angles as indicated on the AI? See, in my hood work, I've
only looked at the pitch on the AI, the TI, altitude and compass - never
thought to look at the angle of bank.

OK, next question... If I'm banking at 20 degrees and moving at 100 kts, I
will turn 180 degrees in a given amount of time and radius of turn. If I do
that same 20 degrees of bank at 60 kts, I will turn 180 degrees in a given
amount of time and radius. I know the radius between the 2 turns will be
different. My intuition says that the turn will take the same amount of
time - the radius is bigger but you're doing it faster. But the ROT I noted
above for a standard rate turn woulf seem to indicate otherwise. I know I'm
missing something but seem to be having a brain fart and can't see it. Was
gonna go up and try this out but noticed my solo endorsement was up on
Friday and my CFI won't be back in town till Monday.

Thanks in advance...