Linux as a learning category covers the OS that runs most servers, embedded devices, and increasingly developer workstations (via WSL, dedicated installs, or macOS as a Unix). The skills split into user-side (shell, package management, systemd basics) and administrator-side (networking, security hardening, performance debugging, kernel internals).
Most courses target one of three audiences: developers who want to be more productive in a terminal, SREs and infrastructure engineers who need to debug production hosts, and would-be RHCSA / LFCS / LPIC certification candidates who need exam-shaped study material. The underlying operating system is the same; the depth and emphasis are different.