I see, you want that that Lamorghini, well if you really want udemy is always a good start. Personally the difficult part for me when learning a new programing language is not resources, it's the motivation to keep do it and I usually need a real project to work on. (10 years + dev)
Usually you find on github "awesome-XYZ" repos (ex: awesome python, awesome c, awesome go), but for cobol, most of the projects are dead
https://github.com/loveOSS/awesome-cobol?tab=readme-ov-file#email
It's a niche, most of the companies use something more modern, easier to find devs, but there are still some that have to mentain that old code while they probably at the same time try to replace it with some other more common language (Java, C++, Rust, Go), I think it's still used by some legacy systems in governments and financial institutions
It's like knowing an extinct language, most of the time is useless, but if someone needs your skills they better pay good for it.