Dual Boot using external ssd

If you have an Intel Mac from 2007 to 2015 (or 2017 for a MBAir), then Linux will run just fine on them even from an external drive.

If you have a 2016+ USB-C Mac, then Linux only has spotty support for the hardware itself....even less support for the 2018+ Intel Macs with the T2 security chip. But again, Linux should run the same whether installed to an internal or external drive. With a 2018+ Mac with T2 security chip, you will first need to modify the Mac's security settings to allow booting from external media as well as selecting the "No Security" option.

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Here are a few articles regarding Linux support on the USB-C Macs (mostly deal with the MBPros)...some of the information in the first two articles may be a bit outdated now.

https://github.com/Dunedan/mbp-2016-linux

https://gist.github.com/roadrunner2/1289542a748d9a104e7baec6a92f9cd7

https://t2linux.org/

At this time Linux support for the M1/M2 Apple Silicon Macs is still under development & in progress...it is not ready to be used yet.

https://asahilinux.org/about/

Keep in mind not all distributions may be compatible with a Mac, or some may need some tweaking & special setup customizations to work. I know Linux Mint and the various Ubuntu flavors work on Apple hardware while Debian installs just fine, but needs some extra custom setup to get all the non-free drivers installed, and may need to modify the bootloader file & folder names on the hidden ESP (aka EFI) partition since Macs have no BIOS configurations to deal with incompatibly named bootloader files/folders like most standard PCs. I have no experience with any other Linux distributions on Apple hardware.

Neither Boot Camp nor Microsoft support installation and running Windows on an external drive.