How do they know that?

I often watch minecraft videos on youtube, how what works, how to build what and sometimes with the command blocks. But sometimes so that the command block changes something in the game, they sometimes give everything some complicated and bad command with numbers and comma without end. How do they know such commands and especially where did they get these information from?

Think with logic…

Maybe. Internet maybe. Programmed yourself

OK. But if I think logically, I still can't come up with a long command that is built from any numbers, punctuation marks or commas

Once you are in the matter you can imagine most of the time. The rest of the time you just look for in the Minecraft Wiki.

Well there's Minecraft Wiki or you have watched YouTube videos yourself or you have a book or something similar for commands and so on

That is normal.
The more a command has information, the longer the command becomes.

Some commands are even nested with multiple execute checks to selectively select multiple conditions.

You can only acquire this knowledge by trying and reading a lot about it.