You know how to add logic on the outside of a function, by putting that function into a larger one and calling the function in the middle.

However, how do you inject logic INTO the middle of a function?

Say you have a function which can iterate over any list and given a condition do a filter. How do you inject the condition logic into that filter function?

In the C days you would use a function pointer for this. C++ introduced templating so you could do this regardless of type. Lambdas make the whole process more ergonomic, it's just declaring a one-shot function in place with some convenient syntax.

In rust instead of the full blown

fn filter_condition(val: ValType) -> bool { // logic }

I can declare a function in place with |val|{logic} - the lambda is just syntactic sugar to make your life easier.

They're just functions that you define inside another "outer" function's local scope, and depending on the language they will/can also work as closures - meaning they capture the values of variable in the outer function as well.

I don't know the syntax of kotlin, but lambda functions generally are usually useful to pass as parameters to other, generic/higher-order functions. For example if you have a sort function `sort(listofitems, compareitems_fn)` you could write the `compareitems_fn` as a lambda directly in-line with the call to sort()