ᐅ Height of cooktop/oven + length of countertops

Do you have them cut to size, or are they sold in these exact dimensions? If so, where? :3

At Hornbach. Kitchen countertop Oxid 34321, 4100 x 635 x 38 mm (161.4 x 25 x 1.5 inches)

According to an employee, this countertop can also be ordered with other finishes. At our store, the usual delivery time is about three weeks.

Speaking of cutting – the niche where the kitchen unit will be installed measures exactly 3068 mm (120.8 inches) wide. Is there any reason not to have the countertop cut to 305 cm (120 inches) instead of exactly 300 cm (118 inches)? (Can you tell this is my first time?)

18 mm (0.7 inches) of air is quite a lot, 2 mm (0.08 inches) for the edge banding, so there are still 16 mm (0.6 inches) left. If 3068 mm (120.8 inches) is the smallest distance in the niche, I would have it cut to 3060 mm (120.1 inches).
How will the wall finish be done?

This wall finishing strip will be used:

Unfortunately, it is not clear from the measurements at which height the width of the niche was taken, so I will probably visit the site tomorrow and measure it myself before relying on the 3068mm (10 ft 1 in).

That definitely makes sense. Niches are often not the same width at the front and back.
Check the profile to see how many millimeters (inches) it can span so that the screws still grip firmly in the panel.