How do I make something like 1,000,000 turn into 1M in my game?

Wait, what is the float expression? How do I use it?

Also, with this float thing, do I use it in a separate event or put it in the same one, and if I do how do I implement it?

I've no idea how to use the left or float expressions.

You could refer to the manual on expressions and system expressions.

construct.net/en/make-games/manuals/construct-3/project-primitives/events/expressions

construct.net/en/make-games/manuals/construct-3/system-reference/system-expressions

Recommend you go through the beginner tutorials as well.