Slax (https://www.slax.org/) runs entirely off of a USB stick. There's other options that have been around for awhile aimed at simple experiences for older and weak hardware.

Back in the netbook era, there were quite a few that tried to be internet browser focused like peppermint OS, though I don't know how well any of them faired with the rise of chromeos.

Is that what a live USB is say with Ubuntu

Not sure if ChromeOS Flex is supposed to wipe the host OS or just run on top of it.

I do not understand this question.

Is there a reason you cannot install Linux ? Linux can be installed from a USB and should work on any system running windows 10.

If Linux is too heavy for your system, there is always NetBSD. I have NetBSD on an AMD 300MHz (= PII) with 512 MB memory and it runs fine.