All addressing security and rendering to different OS's, browsers, and screen sizes in different ways.
I have Googled, watched videos, asked friends, I just don't get it.
i think your research will start yielding "chunks you can digest." If you research these strategies separately: there is no shortage of excellent overviews and tutorials for each of them ... but, they are often changing in ays you must attention to use (like Angular whose latest version often breaks the previous version.
Pick one or two of the "frameworks" that use a language/OS/IDE you are familiar with, and explore those in more depth.