The Linux Foundation has slowed down a little since Covid-19, but they still offer a large number of Free Software education for careers and for fun.
https://training.linuxfoundation.org/
Just a reminder that they are still alive and available.
https://forum.linuxfoundation.org/
Linux Kernel Development
Working on the Linux kernel means building the foundation of everything from servers to smartphones. It’s deep technical work—coding, debugging, and contributing to the engine that powers much of today’s tech.
Introduction to Linux (LFS101x)
Training
Introducción a Linux (LF-UPV-101x)
Training
A Beginner’s Guide to Open Source Software Development (LFD102)
Training
A Beginner’s Guide to Linux Kernel Development (LFD103)
Training
Open Source Software Development: Linux for Developers (LFD107x)
Training
Linux Tools for Software Development (LFD108x)
Training
Git for Distributed Software Development (LFD109x)
Training
Introduction to Open Source Development, Git, and Linux (LFD201)
Training
FOR example...
also - FWIW - O'Reilly, while not specifically a Free Software company, offers many useful training programs and classes.
https://www.oreilly.com/library/view/c20-fundamentals/9780136875185/
for example...
and
https://www.oreilly.com/online-learning/courses.html
Here are some free Classes on Linux fundies
https://www.coursera.org/courses?query=linux
https://www.udemy.com/topic/linux/
Online commercial offerings on Linux training (for money)