Q: Is the sololearn's language "C" an Objective-C?
A: No, C and Objective-C are two different languages. As I understand it, Objective-C was the language used for Apple software development. C language is general purpose and not platform specific (cmiiw).
Q: How I can find objective-C tutorial?
A: You can use any search engine of your choice to find them, just need to be creative with search keywords 👍