The universal basics
Register the drone (if over the weight threshold), pass the basic knowledge test where required (Part 107 in the US for any commercial use), check airspace (controlled airspace needs authorization — LAANC-style apps make it minutes), and keep visual line of sight. Four items, all cheap, all non-negotiable.
Where the rules bite
Over people and moving traffic: no. Near airports without authorization: no. Emergency operations (fires, police activity): absolutely not — drones ground helicopters. Night flying and altitudes above 400 ft have their own rules. The regulations are the first constraint on every flight plan; the battery math is the second.
The commercial line
Any compensation-adjacent flying (real estate photos, mapping for hire, monetized channels) is commercial under most regimes — requiring certification regardless of drone size. The certification is cheap and one-time; the fines for skipping it are neither. The mapping calculator's regulation note exists for exactly this reason.