"questions about imac external monitors

I use usb to connect imac all-in-one, and then usb-hdmi junction box-vga,vga to connect other monitors. Connected to the imac also has a response, but other monitors are still no signal. Is it possible that usb can"t transfer video signals for display?

Jul.26,2021
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-64f5-1b361b6-2bfee.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b361b6-2bfee.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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