Nobody knows everything. We all need to update our skills from time to time. These sites run free courses on writing or related subjects. Some run at specific times, some are always available. As things are updating all the time this is not a list of courses, just a general idea of what's available on each site and a link to explore further and hopefully find something to meet your needs.
If you like your learning with a live human either in person or online Dreena Collins runs classes/workshops and gives talks on writing and/or marketing. See her website for more information. https://dreenawriting.co.uk/author-services/
OpenLearn have a range of free courses which cover various aspects of writing and marketing. Courses are available immediately and you can study at your own pace. There are various ways to find what you are looking for. Courses can be sorted by time and level from introductory to advanced.
The site includes articles, videos and interactives if you would prefer not to do a course.
FutureLearn offer a range of free short courses on writing, and marketing, that are available on certain dates. You can sign up ahead of time if you find something you are interested in doing and they will remind you when it's going to start.
I like the interactive aspect on FutureLearn. I learned a lot from my fellow students.