Hey Vikas Poonia,
Prior to CSS grid/flexbox, to build responsiveness we would be using floats/position hacks to get things right. Since the rise of flexbox and grid its been almost gone now. But it would be good for you to know about that.
Building responsive layouts without using flexbox or css grid. Makes you know what breaks when and why and flexbox and CSS grid are not 100% gone, you might need them sometime. IMO building responsive layouts using Flexbox or CSS Grid is not as hard as using float/position hacks. Anyways, if you want to know more on responsiveness. Google has some nice insights on things like these.
After all of this, I would suggest you to start building basic layouts make them responsive, slowly dig into complex layouts, and make them responsive. Dribbble can be of great help if you want to see how people bring in creative ideas keeping responsiveness in mind too.
IMO, keep the frameworks aside for a while if you want to know what happens under the hood. Once you know how things work you can use anything to get things done.
Hope this helps.