In the frontline or on the frontline?

English as a Second Language English Vocabulary, Grammar and Idioms
1

eg. We produce drugs and medical supplies for injured soliders and civilians [color=blue]on the frontline.
We produce drugs and medical supplies for injured soliders and civilians [color=blue]in the frontline.

Which one is better? And what is the difference between In the frontline and On the frontline?

2

On the front line(s)

3

I can’t tell the difference between them. Why? Could you please help me to clarify them?

4

The other form uses the wrong preposition.