This is so very true. Copy & Paste never made anyone better at anything (except for muscle memory in your left hand). For a while I typed out every single piece of code I used from the internet and it gave me a deeper understanding of what's going on in this code. I reflect on the code while I'm typing it.

I also do this with math these days. Whenever I'm learning a concept I'm writing out all the definitions and theorems, which gives me time to reflect on them and helps memorization. In general, writing is an excellent way of learning.