What distinguishes one-level, two-level, and three-level hardware configurations, and which is production-grade?

In a dynpro, what runs at process before output (PBO) vs process after input (PAI)?

Where does the enqueue server keep its locks?

Where does the message server run, how many exist in a system, and what does it do?

SM66 vs SM50 vs SM51 — what does each show about work processes and instances?

Today, what's the only real difference between the primary application server (PAS) and additional application servers?

What is the ABAP platform's relationship to SAP NetWeaver and to S/4HANA?