I tried to search something about this but found only a small amount of information.
What confuses me is that all isolation amplifiers, optocouplers etc. I could find usually work at much lower frequencies than hundreds of MHz. So it is a mystery to me how the oscilloscopes work.
For example, here is some photos of Hantek DSO1152S https://www.eevblog.com/forum/testgear/hantek-dso1152s-wont-boot-after-firmware-update/100/ and DSO1122S http://www.davidefabbri.net/files/foto/SANY0116.JPG
It is very difficult to understand from those photos how the isolation works. I only could find about DC-DC converters that seemingly used there https://lib.chipdip.ru/503/DOC001503388.pdf