Formbuilder Nested Array Specific Value Change Detection

I am working with Formbuilder and I have an object with two nested arrays (one shown for simplicity). I am looking to use an onChanges() to look for any change to one specific item in the form. I have successfully made it work to detect changes on the first array as a whole, but I only want the function to call if there is a change to a single field.

My form is initialized a such:

this.personForm ={
first: [''],
familyMembers: this.fb.array([this.initFamily()])

familyMembers is:


My changes function is initialized in ngOnInit and looks as follows:

onChanges(): void {
const nameControl = this.personForm.get('familyMembers')
(value:string) => console.log(value)

That code will run when I make any change to a family (name, relation, city). I need it to run only when the city is changed and it needs to return the index in that array.

I am aware that I can write a function to do this and call it via the oncahnge in the form itself but I would like to see if it possible to do it like this.