When rear wheels are stopped transmission is stopped, 0 RPMs
The clutch disconnects engine RPMs from transmission RPMs.
If you are stopped and in 1st gear with clutch pedal down, of course, then clutch disc and transmission input shaft are at 0 RPMs because you are in 1st gear and trans is at 0 RPMs.
The flywheel and pressure plate are at engine RPMs, say 700rpms
There is one supporting bearing that allows input shaft to spin at a different RPM than the engine/flywheel, the pilot bearing, it is in the flywheel/crank shaft.
If it was failing it would tend to make a noise anytime flywheel and input shaft were at different RPMs, so any time clutch was in when shifting gears, not just in 1st.
There is also the throw out bearing, because the slave is always at 0 RPMs there needs to be a bearing that it can push on to push in the fingers on the pressure plate releasing pressure on the clutch disc.
This could be whats squealing.
When the clutch pedal is all the way down there is full pressure on the throw out bearing, when they fail this is often when you hear a noise, but it could also make the noise as pressure is released, so a noise only between full pressure and 0 pressure.
And then there is fluid on the clutch disc, oil or clutch/brake fluid can get on the clutch disc from a leak, this can cause a burning smell and/or a squealing noise as clutch is engaged from a full stop
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