ContentView.swift

import SwiftUI
struct ContentView: View {
  @State private var exercises: [Exercise] = Exercise.sample
  
  var body: some View {
    NavigationSplitView(sidebar: {
      ExercisesList(exercises: exercises)
    }, detail: {
      ExerciseDetail(exercise: exercises.first!)
    })
  }
}

struct ExercisesList: View {
  let exercises: [Exercise]
  @State var activeItem: Exercise?
  
  var body: some View {
    List(exercises, id: \.id, selection: $activeItem) { exercise in
      NavigationLink(value: exercise, label: {
        ExerciseListItem(exercise: exercise)
      })
    }
    .onAppear(perform: {
      if activeItem == nil {
        activeItem = exercises.first
      }
    })
    .navigationTitle("My exercises")
    // Need to use this one so we can get the iPad split view to work with list selection
    .navigationDestination(item: Binding<Exercise?>(get: {
      activeItem
    }, set: { newValue in
      // Use a custom binding to prevent unwanted deselection on iPad
      guard let newValue else {
        return
      }
      
      activeItem = newValue
    }), destination: { exercise in
      ExerciseDetail(exercise: exercise)
    })
  }
}

I made some guesses about the nature of your project, but does this work the way you expect it to? It sets a default item, and it updates the detail with the current selection.

import SwiftUI
struct ContentView: View {
    @State private var exercises: [Exercise] = Exercise.sample
    @State private var activeItem: Exercise?
    
    var body: some View {
        NavigationSplitView(sidebar: {
            ExercisesList(exercises: exercises, activeItem: $activeItem)
        }, detail: {
            if let activeItem {
                ExerciseDetail(exercise: activeItem)
            } else {
                Text("Please select an exercise")
                    .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .center)
            }
        })
        .task {
            activeItem = exercises.first
        }
    }
}

struct ExercisesList: View {
    let exercises: [Exercise]
    @Binding var activeItem: Exercise?
    
    var body: some View {
        List(exercises, id: \.id, selection: $activeItem) {
            ExerciseListItem(exercise: $0)
                .tag($0)
        }
    }
}

struct ExerciseListItem: View {
    let exercise: Exercise
    
    var body: some View {
        VStack(alignment: .leading) {
            Text(exercise.type.rawValue.capitalized)
            Text(exercise.date.formatted(date: .numeric, time: .omitted))
        }
    }
}

struct ExerciseDetail: View {
    let exercise: Exercise
    
    var body: some View {
        Form {
            LabeledContent("Type", value: exercise.type.rawValue.capitalized)
            LabeledContent("Date", value: exercise.date.formatted(date: .long, time: .shortened))
            LabeledContent("Reps", value: exercise.count.formatted())
        }
    }
}

struct Exercise: Identifiable, Hashable {
    let id = UUID()
    var type: ExerciseType
    var count: Int
    var date: Date
    
    static var sample: [Exercise] = [
        .init(type: .pushup, count: 10, date: .now),
        .init(type: .crunch, count: 14, date: Calendar.current.date(byAdding: .day, value: -1, to: .now)!),
        .init(type: .jumpingJack, count: 14, date: Calendar.current.date(byAdding: .day, value: -2, to: .now)!),
    ]
}


enum ExerciseType: String {
    case pushup, crunch, jumpingJack
}