the RTH height is relative to the take off point.
So if you set the rth height at 70ft that is 70ft above the take off point, it does not matter what the ground below the drone does the RTH height remains 70 ft above the take off point.
You could fly down the falls and end up 100's of feet below your take off point and trigger an RTH, Without pilot input the drone will climb, vertically, to a height of 70ft above the take off point before starting to move towards the homepoint.
There is a problem in this in that if there is something like a tree, a building or a hill between you and the drone that reaches to more than 70ft above the take off point the drone will fly into it during RTH. That's why it is important to check that the RTH height is relevant to each flight.
If your drone is ABOVE the RTH height when an RTH is triggered it will remain at its current height and return home if it can.
You can change the RTH height during a flight BUT I do not know if the change will take effect if the drone is already executing and RTH.
Also BE AWARE that if the drone is within 20m of the homepoint when the RTH is triggered the drone will ATTEMPT TO LAND WHERE IT IS.
The RTH section of the manual
https://dl.djicdn.com/downloads/Mavic_Mini/20200212/Mavic_Mini_User_Manual_v1.0_EN.pdf
starts at the bottom of page 12.
In a safe place during low and slown practise sessions I would strongly suggest deliberately causing or triggering all the RTH behaviours BEFORE you encounter during a real flight