I plan to sell course(s) on Udemy by using Ubuntu. As you predict, there will be everything what the Ubuntu has (such as screens, icons, logos, symbols, tools, softwares etc. and their use) in the course videos.
However, I want to know this is legal or not.
An answer is suggested but I don't think that it is enough for my question because I can't see a clear definition such as "Ubuntu is under GPL", "Ubuntu is open source", or "Ubuntu is free" on ubuntu.com. Therefore, I can't determine an answer for the question I asked.
Definitions on https://ubuntu.com/licensing is another cause to this question:
Ubuntu is a collection of thousands of computer programs and documents created by a range of individuals, teams and companies. Each of these programs may come under a different licence.
Ubuntu contains licensed and copyrighted works that are not application software. For example, the default Ubuntu installation includes documentation, images, sounds, video clips and firmware.
All of the application software installed by default is free software. In addition, we install some hardware drivers that are available only in binary format, but such packages are clearly marked in the restricted component.