I had a designer create a custom sysmbol using Apple's SF Symbols documentation. They gave me an .svg file that I dutifuly added to the asset catalogue (again using Apple's documentation) but when I tried using it in a view it does not appear to be rendering as expected so is there some other step required to make these things work in SwiftUI?

How is it rendering?

All you have to do is import your properly formatted SVG (and "properly formatted" is very important) into your assets catalog, then you can use it in your code like this:

let img = UIImage(named: "yourImageName")

You can adjust various properties of the image using SymbolConfiguration, as per Apple's docs.

For mine, I created an extension on UIImage to make it easier to create an image in different scales.

extension UIImage {
    static func catHeadImage(filled: Bool = false, scale: SymbolScale) -> UIImage {
        let config = UIImage.SymbolConfiguration(scale: scale)
        return (UIImage(named: filled ? "cat.head.fill" : "cat.head")!.withConfiguration(config).withRenderingMode(.alwaysTemplate))
    }
}