I've been learning Swift for the last couple weeks on the weekends and I understand, from what I've read, that functions are basically closures. But, this confuses me because obviously a feature thats 100% redundant wouldn't be implemented, so when should you use closures vs functions? Are there times you can only use closures in Swift, or can you always use them interchangeably?
They are not totally redundant. Major difference is a convenience of writing and the capability to name parameters or the func itself. This improves largely readabilty of code.
We could say closures are a stripped down version of func. But they are functionally equivalent.
See more details here: https://medium.com/@anuj.rai2489/closure-vs-function-vs-protocol-in-swift-94d0ecd03406
Each time you have a closure you can put a function. If code is called only once, it may not worth using a function rather than just writing the closure. If the same code is called many times may be a function will be better for maintenance.