im a beginner and slowly learning thru youtube.

i have a question, this thing they do in mangle, is it a thing?

in mangle, for browsing
this is only sample,
small = mark connection connection bytes = 64k-512k protocol = tcp port=443,80
medium = mark connection connection bytes = 512k-5m protocol = tcp port=443,80
large = = mark connection connection bytes = 5m-0 protocol = tcp port=443,80

they said this is better than combining them all as browsing? is it better to seperate them?

the one reason you need to split traffic classification like this is for assigning different priority in qos/queue, like to set smaller bytes get higher priority over higher bytes. usually you do this to separate browsing traffic & download traffic.
if you do not need to change priority between packet like that, better use single entry and queue with burst so small bytes can have higher bandwidth while bigger bytes has lower bandwidth.