What does the slash "/" at the end of the URL mean, or is it missing?

Question

URL Handling

Understanding the docsify documentation generator.
In short, about the paths: if you create a file in the root of the "project", for example guide.md, then you can go to reading it using this url - domain.com/guide . If you go along the path domain.com/guide/ , then you will get a 404 error.
If you create a folder in the root of the "project", for example ru-RU and the README.md file in it, then you can go to reading it using this url - domain. com/ru-RU/ . If you follow the path domain.com/ru-RU , then there will be a 404 error.
https://docsify.js.org/#/more-pages
That is, as I understand it, if there is no "/" at the end of the URL, then the server looks for a file with the corresponding name and .md extension, and if there is "/", then the server looks for a subdirectory with that name and a README file in it. md
Considering, for example, the Youtube link system, the link https://www.youtube.com/c/Microsoft/ and https://www.youtube.com/c/Microsoft display the same content.
Why in some cases "/" at the end affects something, and in some cases - does not affect?