RWD: CSS FlexBox vs Grid vs Float

Question

When designing a Layout what will be the approach?
CSS FLexBox
CSS Grid or Float.
For instance if I want something like this:
Where number represent div or containers
Desktop :
***************************
* * 2 *
* 1 * *
* *****************
*********** 4 * *
* 3 * * 5 *
***************************
on iPad and tablets
***************************
* 1 * *
* * 2 *
***************************
* 3 * 4 * 5 *
***************************
and stack on top of each other when on mobile phones
NOTE: the slight difference between desktop and tablets layout.

Answer 1

@Tim Millar Yeah I've used it for some projects, but I want to know how to build RWD without the need of frameworks, at the exception of Normalize which just set the standard display for some html elements.
I really get confused when using flex, same with float, I don't know anything about grid but I heard it's the new comer.
Bootstrap seems to use float but trying to understand the code is overwhelming.

Answer 2

Is bootstrap an option?