Car will shut off while at a light or driving?

Hello,

The engine stalling sounds like it could be due to a faulty crank position sensor.
Overtime with heat and vibrations from the engine the crank sensor can loose its calibration and can send incorrect signal to the ECM. And that can trigger the engine to stall.
It can also produce no start issues.

https://www.2carpros.com/articles/engine-stalls

Sometimes this will trigger the check engine light. Is your engine light illuminated?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

You said that you have to add coolant. So is there a leak in the cooling system somewhere?
Does the engine overheat before shutting down? If it overheats than the shut down can be due to the coolant temperature sensor signaling the ECM to shout down due to overheating.

https://www.2carpros.com/articles/symptoms-of-an-overheating-engine

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.