Bootstrap. How to pin navbar on top and align content to center?

Question

css

I spent a lot of time and did not understand how to do this, because I am new to the web. I need the content to be center aligned as it will have some height (not hardcoded, will depend on the content). If the user reduces the height of the window, then the content will also be centered until it hits the navbar and a scroll bar appears near the window.
Here is what I made.

Answer 1

Learn the basics.
https://www.w3schools.com/css/css3_flexbox.asp
https://css-tricks.com/snippets/css/a-guide-to-flexbox/
https://medium.com/@stasonmars/%D0 %B2%D0%B5%CC%88%...