Boot from SD, run from external (USB) drive

I recall seeing something related to this but I can’t find it again. Can you guys point me toward a simple howto, please? Did it many times on the RPi, but dang, with 13 weird partitions I’m having no success. What are those partitions about, anyway?

I’m assuming once booted and switched over, I can rcp back onto the SD card for a running backup, or is that not happening? Even if we could mount the usr partition on the SD card from the USB drive?

Btw, just for this experiment, I got a USB 3.1 Gen 1 128GB SSD for less than $30 on Amazon; 512GB is less than $65, IIRC.

Boot from external usb drive is currently not possible for nano.
Please refer to this thread:

No, the idea is to boot from the SD drive, as normal, but then mount the external SSD as the normal working drive(s.)

Follow this tutorial , very easy -->

Can any of the movement and resizing tasks be performed by gparted?

Of course you can try parted, they are equivalent.
However NVIDIA use same sgdisk command in their script file.