If you'd like to do some light gunsmithing on YOUR OWN guns for fun, and have plenty of money to burn, the online or correspondence courses can at least get you started.
If you intend to make money doing gunsmithing these courses are hyper-expensive wastes of time and money.
You can't learn highly technical skills like this at a distance, you need to have a well-trained instructor looking over your shoulder and prompting you on how to do it.
If you want to try to self-teach, buy some of the Jerry Kuhnhausen Shop Manuals.
These were written as training aids for new gunsmith students.
Each manual fully covers a specific type of firearm and shows how to gunsmith the FACTORY way.
These will not show the old gun plumber methods of making parts or how to somehow get a gun working even if it actually damages it.
Just pick manuals for guns you own, sit down and STUDY the manual.
Kuhnhausen figured his students would have enough brains to actually spend time reading and wouldn't need huge letters with arrows pointing to critical data.
After studying the manual, disassemble a gun of yours and use the manual to learn how it works.
The rest is learning how to use small tools.
There's this picture of a real gunsmith standing over a large milling machine or lathe making a precision part.
The fact is, MOST of a gunsmiths time is spent sitting at a bench with screwdrivers and punches working on a small assembly.
The Kuhnhausen Shop Manuals are the best reference there is.
https://www.brownells.com/search/index.htm?k=shop+manual&ksubmit=y
Midway also sell the accompanying videos.
https://www.midwayusa.com/kuhnhausen-books-and-videos/b?bid=1195