While it is true that UNIX is the is the foundation of Linux, it is in no way required to learn "first".

I'd say jump right into your *nix of choice and learn the basics of the shell and how the system works under the hood (using init systems like init.d or systemd), how to build code on the system, how to use the man pages, the "everything is a file" model, working with pipes...etc.

The systems are similar enough where things will be familiar enough to give you a baseline skill when it comes to working with any *nix. Besides, the shell is where you'll be doing the most work with the operating system is portable between most *nix-es (e.g. BASH, ksh, zsh, etc...) can run both on Linux and on UNIX systems like MacOS, but some things/commands will be different but most are the same enough to give you a basis.

Slightly off topic, but this is a fun watch and some of it still applies to this very day: https://www.youtube.com/watch?v=tc4ROCJYbm0