You can design for devices but the current consensus is that you change the ‘layout’ when you feel it’s ‘broken’.
If you add the media queries (google it) when your design needs it it will, generaly, fit the device being used.
There are kind of ‘standard’ widths you could use as guidelines but that’s up to you.
And how do you judge how many % a width is if you don’t know the size of the user?
I don’t need to know the user’s screensize.
If I want an element to be half the screen wide, I use 50% and so on and it just works. If I absolutely need an element to be not less than a specific px value, I can use
max-width and a percentage width and it all fits nicely.