Here are couple of options for online training that I would recommend.
Linux full course: (YouTube) about 7.5 hours using Ubuntu, Zorin, etc.
Link - The Complete Linux Course: Beginner to Power User! - YouTube
Linux crash course: (Udemy) about 13.5 hours focusing mostly on CentOS
Link - https://www.udemy.com/course/learn-linux-in-5-days/
So, it would depend on whether you want to take the Red Hat path or the Debian or even one of the other options such as Arch, Suse, etc.
Either one (Red Hat or Debian) will be about the same, "generally speaking" it’s just a matter of using Yum or apt \ rpm or deb commands \ packages respectively for Red Hat and Debian.
As for the real world difference between the two paths, the Red Hat path (Red Hat, CentOS, Fedora, etc.) are "more commonly seen" in the industry side, NASA, Dell Computers, IBM, etc. while the Debian path, (Ubuntu, Linux Mint, Zorin, etc.) are more commonly used by the end-users on their personal home PCs \ laptops.
Please note the emphasis on "generally speaking" and "more commonly seen". It's not to say that the Red Hat line is only used in the industry and the Debian line is only used on the personal PC side, quite the opposite, I have seen both OS lines on both sides of the computer world. These statements are just a generalization.
Hope that helps.