Comment by MisterTea

Those are compression fittings which we also use in the USA for sink and toilet connections. Standard size here is 3/8 inch which is the outer diameter of the tubing. They are easy to work with as you only need a tube cutter (or hacksaw with fine tooth pitch) and a wrench. You cut the tube, slide a compression nut over, slide the ferrule(s) on, insert the tube into the fitting, finger tighten the compression nut, make sure everything is aligned then tighten nut with wrench. No solder, no fancy crimp tools. You can disassemble and reassemble without leaks. The only issue is the ferrule winds up becoming swaged to the tube after wrench tightening so you have to cut the tube end off behind the ferrule if you want to replace the nut.

However, instead of using rigid metal or plastic tubing you commonly find a flexible hose assembly[1]. Only fancy installs use custom formed polished rigid tube for the look. We also tend to leave most of the plumbing in the walls and only the shut off valve protrudes from the wall. Maybe there are flexible hoses available in your region.

When I installed my bidet all I needed was a 3/8 compression tee[2] which they make specifically to tap shutoff valves. The bidet came with a 3/8 connection hose so that went to the tee and I was up and running. The big issue was electric - no outlets in the bathroom. So having seen how Japan loves extension cords I too used one, ran a 12 AWG cord out to the hall to a GFI adapter plugged into a socket. Jank but highly unlikely to cause a fire.

[1] https://www.homedepot.com/b/Plumbing-Plumbing-Parts-Supply-L...

[2] https://www.amazon.com/Breezliy-Lead-Free-Compression-Outlet...