Persistent USB is new thing to me. I’ve found several places where it is discussed how to make and amend one. https://askubuntu.com/questions/1181854/how-is-it-easier-to-make-a-persistent-live-drive-with-ubuntu-19-10 https://askubuntu.com/questions/1189020/how-can-i-get-a-live-only-drive-with-ubuntu-19-10-and-newer-versions https://askubuntu.com/questions/922692/how-to-edit-a-bootable-usb-stick.

What seems strange to me that I have USB with Linux Mint and also

partition which is /dev/sda3 when I insert USB and it contains folders named
install logs date-time ...
. But when I boot from USB and check I see:

mount | grep sda3
mount /dev/sda3 /var/logs

but /var/logs looks ordinary, there are no

install logs date-time ...
folders. When I added a file to /var/logs I then found it in new
install logs date-time ...
when booted "normally" and inserted USB.

How can is be done that folders at root of a partition are hidden when mounted when booted from USB cd partition?