How is mobile resolution calculated?

My phones resolution is 720x1600, but the resolution in Roblox is somewhere around 829x348.

This in itself isnt a problem, since my phone already struggles with running Roblox games, so a bigger resolution would be a horrible idea.

What Im asking is, how is this calculated? Ive heard a staff member say that its a quarter of the original resolution, but 1600 / 4 is definitely not 829.
Im sure it has something to do with DPI or PPI, but I dont know the exact calculation. I want to make some custom emulators for newer phones but they will be very inaccurate if I use the “actual resolution”

EDIT: It seems like its actually a quarter of the total pixels. So 1,600 x 720 = 1,152,000 and 829 x 348 = 288,492. 1,152,000 / 4 = 288 000. Thats close enough, but still dont know how to get an actual resolution from there.