The simple answer is… …the plot. There are many articles on the net about writing ‘stong’ female characters. Many point out that male main character are assume to be strong because they don’t need to be specifically described as ‘strong’.
One of the better articles talks about female characters needing agency to qualify as actually being strong. It’s simpler than that, they need agency to qualify as being main characters. Otherwise they are just scenery.
What I look for is characters that make things happen. A main female character should be at the crux of the choice in the inciting incident that kicks the whole conflict in a story off. Even better their decisions should turn the conflict at the midpoint and even drive the conclusion. If this happens the female character can be considered ‘strong’, otherwise, she’s decoration.
Recently there’s been a trend of movies having tough woman but if the character is not making decisions based on her own criteria and motivation, how can she be strong no matter how much ass she kicks? Is there really any difference between a woman cleaning out a den of villains or emptying out a load of laundry if she just does it to please her man? Really?