"Where and how should I write the constants that can be used throughout the application?

If you want to create a special class, I think you should use structure.

Swift's class cannot have a class constant to hold the value, but it is possible if it is struct.

// Compile without problems
US>structure ConstStruct{
    static let nameMax = 32
    static let imageMargin —CGFloat=10.0
    staticlet defaultName="NO NAME"
}

// get a compilation error
classConstClass{
    static let nameMax = 32
    static let imageMargin —CGFloat=10.0
    staticlet defaultName="NO NAME"
}

"If you give ""let"", I think you can treat anything as a constant.
If you don't forget the style of C language, you may not be able to feel it right.

As for "where", I think it would be independent of the implementation of classes, structures, etc.