Why do gears slip on a bike? Rear Derailleur Installation

In this video tutorial I will show you how to install and adjust a rear derailleur.
You will find out why your bike shifts badly.
For example, we will use a bicycle "Cube".
The "Cube" bicycle has a defect from the factory. We will fix this defect on the bike
This video instructions "diy" for cyclists. Video instruction in the style "asmr".
It will be interesting for bikers!

00:00 Bike not shifting gears
00:23 Front derailleur shimano slx
00:45 Bike rear wheel installation
02:07 Bike rear derailleur installation
02:55 Bike shift cable repair
08:00 Unpacking sram 9-speed chain pc 971
08:34 Bike chain sram installation
10:18 Bike rear derailleur adjustment
12:44 Why do the gears on my bike slip?
13:03 Installing a new rear derailleur cable cover
20:50 Rear derailleur adjustment
23:57 Good result