If the front and rear ends are separated, how does nginx obtain the front-end ports?

problem description

when a page running by npm run dev sends a request, the $remote_port of nginx is a random port (refresh the page, the port number changes), so how does nginx get the port number of the front end?

Nov.09,2021

get request header / contains host related information
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e582d7-4548c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e582d7-4548c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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