the power of flex and grid that you do not need to use media screen and those stuff with it so for example you just set how it wrap when there no enough space same thing for grid
i know there some compatibility with gird layout with different browser s check https://www.caniuse.com/css-grid to know which option you can use with each browser
do not try to develop site that compatible with everything cause that sooooo hard and in many case there no point to make later there will be lesson teach you how to balance between use new option/feature and make the site compatible with old devices and browser and there some site can check that for you and even give you fallback code to help you
so wait till you finish the js course as some of the polyfill if not most use js code instead of unsupported css feature
and those lesson give you some general idea about what you will face and how to find your way to solve it