In how many ways can one select 3 pens out of 10 pens?

If you just answered C(10,3), you have conceptual issues from basic Combinatorics. If you answered 1, you know the basics.

What is C(n,r)? Why is it similar to a 2D square maze made up of square tiles? Why is C(n,r) a Pascal's Triangle?

If know don't know the answers to these, you need extensive research level experimentation and elaborate observations. Don't mug up (a + b)² = a² + 2ab + b² but should know why we have 3 terms and why is the coefficient 2 for ab.

Combinatorics isn't "knowing how to do it", it is usually why to use what and where. You would need to deep dive into Binomial and Multinomial first. Please refer YT videos, math forums like (stackexchange and brilliant) and check out some freely available videos by some Institutes.