You probably have come across https://www.w3schools.com/
Its newbie friendly documentation on vast number of programming technologies, which doesnt dwell in too much specifications and have handy examples. As a newbie, whenever i came across a line i didnt comprehend, id look it up there. Stackoverflow is also great to find the answers of common and not so common problems. Usually whenever i put s specific question onto google, ill have a useful link among the top results.
About multi platform responsiveness, im not so sure, i dont really focus that much into making my projects answer to mutli browser specifications, for now i focus they work alright on chrome. If you havent figure out yet, you can use the
inspect option when clicking right button over your web page in chrome and you can see feed from the html code and console. You can also adjust the screen size, which simulates different ratios, its good way to see if your page looks alright on phone lets say.
Flexbox has its own flex properties(on child items), grow shrink and basis, and i admit even i struggle still to fully comprehend them, but usually you can make your elements align alright using them. For example with flex-basis of 50% the element will take up to 50% of the container row(when its direciton is row), which means 2 elements will fit into 1 row and other elements will go next row, but again, this also takes in mind other properties, like margins, etc. There isnt really a right way to do it, only a way that fits the current situation and your current foresight of the project. Your job is to figure out how to make it happen . Max width can also come in hand