here is an article that may help clarify, at least, some of your questions.
www.futurehosting.com/blog/web-design-ba...ing-elements-in-css/
I have quoted some of the basics points below:
...Pixels are absolute but not consistent.
In practice, they aren’t the same length everywhere because different devices treat them differently, but on each device a pixel is always the same. 16px on your laptop monitor is not the same as 16px on your iPad...
...Ems have a problem. Because everything is sized relative to its parent element, the meaning of an em changes as elements are nested. ..
...Rems, root ems, are always relative to the font-size of the <html> element. It doesn’t matter how deeply nested an element is, its rem lengths will always be a proportion of the font-size of <html>...
...Rems are awesome in theory, and mostly awesome in practice. There are some cases where simple proportional scaling doesn’t work out quite right — some text gets too small or too large relative to the surrounding text...