Came across this doc for the extroot multiple partitions, question, does it bring any benefit to have the swap partition on USB drive? And How does the extroot know the individual partition size that you want to create?
mkfs.ext4 /dev/sda1
mkfs.ext4 /dev/sda3
yeah it help having swap on extroot USD drive as you don't want it on your flash to wear it.
for creating such partitions you should follow official openwrt docs like https://openwrt.org/docs/guide-user/additional-software/extroot_configuration