Which power supply should I use (USB vs external), and is it safe?

For most beginner projects, USB power is totally fine. Problems start when you add motors, servos, long LED strips, or multiple modules that draw real current.

When USB power is enough

Blinking LEDs
Reading sensors
Small OLED displays
Basic serial debugging

When you should use an external supply

Motors and servos
High brightness LED strips
Relays and inductive loads
Anything that causes random resets

Safety rules that prevent damage

Share ground: if you use two supplies, connect GND to GND.
Do not pull motor power from the board 5V pin unless the project is tiny and the board/regulator is designed for it.
Use the right voltage for your board and modules (3.3V vs 5V matters).
Add a capacitor across the motor/servo supply near the load if you see resets.

Bottom line

USB is great for learning and low-power builds. Use an external supply for motors and higher loads, keep grounds shared, and follow safe wiring to avoid resets and overheating.