What happens if RTH encounters a restricted area?
Will it bypass it, will it stop, or will it go through?
I'm asking this because I thought that if it stops and I have lost signal I might lose the drone.

Hi there. Thank you for reaching out. If the aircraft flies into a Geo zone during RTH, it will either descend until it exits the GEO zone and continue to the HomePoint or it will hover in place. Just a friendly reminder to avoid flying near the GEO zone as it might affect the RTH. Hope this helps. Thank you for your valued support.

If, in an RTH, the drone is approaching an air corridor some drones now have the ability to reduce their height and fly underneath the corridor but they do not climb to RTH height after they have passed the corridor, so be careful.

The manual of the Air 3 contains the following, on page 51 in the RTH section.
" If the max altitude is set below the current altitude during RTH, the aircraft will descend
to the max altitude first and then continue returning to home.
• The RTH Altitude cannot be changed during RTH."
The mavic 3 manual contains the same and this may be what covers the above behaviour although the two quoted sentences appear contradictory.

Suren documented a mavic 3's behaviour, in a thread concerning, I think, one of the DJI mini series drones and in that thread the mini took off under an air corridor, it was flown out to sea and beyond the side of the air corridor. At somepoint an RTH was initiated and the mini ran into the side of the air corridor and got stuck, over water.

Once the mini reached the forced landing battery level it descended. When the mini was below the level of the floor of the air corridor the pilot was able to get the mini over dry land and safe.
The flight log showed a "max altitude" setting of, I think, 60m whilst under the air corridor but an RTH height setting of, perhaps, 75m. That should be impossible. I think the pilot mentioned their set " max altitude" and that it was, perhaps, 80m so it seemed there was an error in the log.
But when the mini flew out from underneath the air corridor the log showed a " max altitude" setting that was equal to, or above, the set RTH height.
With respect to "max altitude", the flight log of Suren's drone showed the same behaviour, but it also shows the drone descending, without Suren's throttle input, as it approached the air corridor and then passing under the corridor.

This is why I think the quote from the air 3 manual is relevant.