Section 3: zkSNARK Theory (Very Math-Heavy)
This section touches on the mathematics behind zkSNARKs, a powerful new zero-knowledge gadget first invented in 2012. zkSNARKs first became viable in production in blockchain systems in the last few years; they are something of a "cryptographic nuke." You should only consider this track if you're completely familiar with everything in Section 2-—I'd recommend not working on this section unless all of the material in Section 2 is very familiar to you.
What are zkSNARKs? (and the linked SNARKs Explainer Series)
An approximate introduction to how zk-SNARKs are possible
Vitalik's Series on SNARKs: part 1, part 2, part 3. Note that this is very technical!
Understanding PLONK: Another more recent zkSNARK protocol.