Run a cable if possible.

Or use wifi but place the wifi access point or antenna, weatherproof units of course, outside the apartment. If you have windows on the same face of the building then it's no problem to main line-of-sight. 3 floors vertically is less than 40ft so it's not a problem.

Can't do much indoors as wifi will have a really hard time going thru 3 floors of reinforced concrete.

Yes, cable between 8th and 11th, one end is UniFi router, other end can be a switch or wifi access point.

Nope. one end will need to have 1 x modem and 1 x wifi router, both supplied by TM. Then you pull a cable from this router to the other apartment and on that end you have another wifi router except this one won't be doing any routing. It'll just act as a wifi ap and switch.