If you want to become an experienced programmer in a new language you have to pick up the big thick books and apply what you learn while reading them. Online courses are nice but they do not go into the depth that the 700+ page books go into. If you go by courses alone you will always be one of those engineers that has a spotty base foundation and will never truly become an expert due to missing out on something huge that was not covered in an online course.
If you want a good course go for the courses that have 35+ hours of content and go through everything. Remember you are a beginner in the new language so skipping anything is not advised as you might miss out on something important that the experienced engineer is laying down in the course. They normally learn the material they build from the courses from the big books and take bits and pieces of what the books have to condense them down into something a little more light weight.
As when you are sitting at the day job and have over 1 million lines of C and C++ code to go through you really have to have a good grasp on the basics and hard parts of the language to be really effective. Being a junior engineer on the most important project a company has requires some serious reading and implementation time on and off the clock to understand what is going on.
The best part is if the more you dive into the big books the faster you will be able to read them and apply what you learn and profit from that new knowledge. Things really come together when you are able to use knowledge of the base languages (C, C++, Rust, Go, etc.) to help troubleshoot problems in the higher level languages that may pop up as engine errors (PHP, Perl, Ruby, Python, Java, C#, etc.)
Just remember there is no shortcut to becoming a truly experienced engineer. Some point you will have to pick up the big books and dive deep and implement what you learn. Those that do not do it normally reach a plateau in their growth until they take the plunge into the big books, code/developer documentation, etc. and to understand it you have to also implement it and work with it on a daily basis.