Explain how to choose the width when designing for an application?

Question

Design

Previously, when designing applications or websites, I used a width of 375.
But now smartphones have become larger, someone advises drawing under 650 and more.
But I can’t imagine how to design for 650, it’s a huge width and it’s hard to imagine, if you look at it at 100% scale, how it will fit into a smartphone, even holding the same iPhone 8 plus in your hands, it seems much smaller visually, than 100% width in the graphics editor.
Might be worth looking at on a smaller scale. Or am I missing something?
Please explain the process.

Answer 1

Perhaps they meant not 650, but 750 - this is just doubled 375.
Since there are practically no non-retinal phones left, everyone draws layouts in double (and in some cases even triple) size.
But the logical width of the screens is not particularly growing, all the same 320-400 are relevant and will remain so for a long time.

Answer 2

Draw in 320, it's a 100% guarantee that your layout will fit on any device.