@Albuquerque,
For a scientist or mathematician, the word "understanding" means being able to use theories or principles to get the correct answer.
For example, you talked about "uncountable numbers" and referred to the "size of a number". What you said made no sense... but I think you are referring to the size of sets, and whether a set is countable or uncountable.
If you took a math class, you would take tests to prove your understanding of each concept. By solving actual problems, you would be able to demonstrate your understanding. More important, when you have a misunderstanding... you could see you go the wrong answer and correct your mistake. This is how you learn when you get the wrong answer.
On a math test, you may be given a number of different sets. For example I could define a set as [pi, pi/2, pi/3 ... pi/n]. I think you would know that each member of this set is irrational.
But can you tell me the size of this set? And can you tell me if this set is countable or not.
If you can't answer basic questions about the terms you are using,
then you don't understand them. If you then encounter these terms in Youtube videos, you are very likely to get a bigger misunderstanding on what the video is actually saying.
The way you get an understanding in math or science is to start with the basics. If you don't understand the basics, it is impossible for you to understand the more advanced topics.