This question have been asked many times on this forum, so i'll just give short answer. I'm just a geek though, not actual expert/professional.
1. Where should I begin if I want to understand how Bitcoin works at a technical level?
The common suggestion is to start by visiting "learn me a bitcoin" website[1] or reading Mastering Bitcoin book[2]. Both of them are available for free and legal.
2. Where do developers and protocol researchers begin??
3. What materials, or hands on projects can I use to understand things like Bitcoin Core, scripting, consensus, mempools, etc.??
Jameson Lopp website have long list of technical information[3]. If you're looking for discussion or progress of certain topic, Bitcoin Optech[4] is also decent source.
[1]
https://learnmeabitcoin.com/[2]
https://github.com/bitcoinbook/bitcoinbook[3]
https://www.lopp.net/bitcoin-information/technical-resources.html[4]
https://bitcoinops.org/