I studied some number theory basic topics and after that I found some topics in some Youtube playlists such as:

• Euler totient
• Möbius inversion
• Chinese remainder theorem
• Diophantine equation
• Linear/segmented sieve
• Miller Rabin primality testing
• etc.

Are these topics really important and frequent in problems with rates $$$<1800$$$ and worth time or it's just a waste of time to learn them now?