I have these two arrays:

var customers = ["Example", "another"]
var projects = [["Website", "Example", "0:34:23"], ["Logo", "another", "0:58:56"], ["Big Project", "another", "5:23:56"]]

How can I make out of this an expandable list, which has in this case two categories "Example" and "another" and the children are the projects where the second item matches the customer.

The endresult should look like this:

Not sure but this might be what you looking for

struct Data {
    var text: String
    var description: [Data]?
}

struct ContentView: View {
    let data = [
        Data(text: "Example", description: [Data(text: "Website"), Data(text: "Example"), Data(text: "0:34:23")]),
        Data(text: "Another", description: [Data(text: "Logo"), Data(text: "another"), Data(text: "0:58.56")]),
        Data(text: "Project", description: [Data(text: "Big Project"), Data(text: "another"), Data(text: "5:23:56")])

    ]

    var body: some View {
        List(data, id:\.text, children: \.description) { item in
            if item.description != nil {
                Text(item.text)
            } else {
                Text(item.text)
            }
        }
    }
}

Give this

or you can look at How to hide and reveal content using DisclosureGroup

Ok thanks i will look at it tomorrow. (I live in Germany)