LinkedIn Courses
LEARNING PATH: Become a C++ Developer, 8 courses and 19 hours of video, probably a good basic grounding in C++.
LEARNING PATH: Advance Your Skills in C++, 8 courses and almost 24 hours of video, this actually includes 3 courses covered in the Become a C++ Developer Learning Path so if you do that Path first, you can cover this path with 5 courses and around 12 hours of video.
COURSE: C Essential Training, it can be quite difficult to pick C courses because of the number available and also because courses tend to be more geared towards C++ or C# which tend to be more popular nowadays but this one looks like an interesting course. The lecturer is Dan Gookin, the author of C for Dummies which is one of the first programming books I ever bought.
COURSE: Advanced C Programming, also a Dan Gookin course, covering more advanced topics
COURSE: Mastering C Language Pointers, this is also a Dan Gookin course covering some more advanced topics.
COURSES: This is a list of courses hosted by Dan Gookin who seems to be the go-to person for C courses on LinkedIn Learning. These are mostly C courses, including the two linked to above but there are also courses on PC Maintenance, Troubleshooting and a few others.
Documentation
Arguably, the best references for C (and maybe C++ as well but probably not C#) is available in book form rather than online. That being said, there are lost of online resources relating to C including a C language reference at cppreference.com which, in spite of the name, is a reference for the C language, not C++. It does also have a C++ Language Reference.