There are plenty of Linux distributions that are meant to run as "Live Operating Systems" on an external flash or optical disk (like BlackArch, Kali, and TailsOS for the infosec community). Performance is degraded, sure, when running an OS like that...however, you could simply make a new partition on your internal HDD from within Disk Utility, and install a Linux distro on that. I installed Lubuntu that way on my MacBook, and it works just swell. Easy steps:
Create a new partition your internal drive in Disk Utility (make it 32gb or whatever you deem acceptable, per your own requirements and/or system limitations, although an absolutely bare minimum of 8gb is needed for most Linux distros)
Download balenaEtcher (https://www.balena.io/etcher/) to flash the installation media to an external USB drive
Download whichever Linux distribution you'd like install as dual boot
Use Etcher to burn your installer to the USB
Reboot your Macbook while holding the 'option' button until you're presented with boot options, then select the USB drive as your boot disk (probably named EFI Boot, maybe something else, but something that's not the Macintosh HD option)
Follow the Linux installation instructions, making TOTALLY certain you target the newly-created partition as the destination disk (so, whichever partition is roughly the size of the one you had just created in Disk Utility in step one)
Note: The installation media you created can, in many cases, be used for Live OS operation. That all depends on which Linux distro you select -- Ubuntu does, and so does it's descendants, Lubuntu and Xubuntu, all of which are descendants of Debian. I'm sure there are plenty others.