ContentView.swift

//
//  THIS DOESN'T WORK, see the final comment for fix
//

import SwiftUI
import AVFoundation

struct ContentView: View {
    @StateObject private var cameraManager = CameraManager()
    
    var body: some View {
        ZStack {
            if cameraManager.isSessionInitialized {
                VideoPreviewView().edgesIgnoringSafeArea(.all)
            } else {
                Text("loading cam")
            }
            RecordButton()
        }
        .environmentObject(cameraManager)
    }
}

class RecordingHandler: NSObject, AVCaptureFileOutputRecordingDelegate {
    func fileOutput(_ output: AVCaptureFileOutput, didStartRecordingTo fileURL: URL, from connections: [AVCaptureConnection]) {}
    func fileOutput(_ output: AVCaptureFileOutput, didFinishRecordingTo outputFileURL: URL, from connections: [AVCaptureConnection], error: Error?) {}
}

class CameraManager: ObservableObject {
    var captureSession = AVCaptureSession()
    @Published var isSessionInitialized = false
    private var movieOutput: AVCaptureMovieFileOutput?
    private var recordingHandler: RecordingHandler

    init() {
        self.recordingHandler = RecordingHandler()
        DispatchQueue.global(qos: .background).async { [weak self] in
            guard let self = self else { return }
            
            guard let camera = AVCaptureDevice.default(for: .video) else { return }

            do {
                let input = try AVCaptureDeviceInput(device: camera)
                self.captureSession.addInput(input)

                self.movieOutput = AVCaptureMovieFileOutput()
                if self.captureSession.canAddOutput(self.movieOutput!) {
                    self.captureSession.addOutput(self.movieOutput!)
                }

                // Start running the capture session on the background queue
                self.captureSession.startRunning()
                DispatchQueue.main.async {
                    self.isSessionInitialized = true
                }
            } catch {
                print(error.localizedDescription)
            }
        }
    }
    
    func startRecording() {
        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let outputURL = documentsDirectory.appendingPathComponent(UUID().uuidString).appendingPathExtension("mov")
        movieOutput?.startRecording(to: outputURL, recordingDelegate: recordingHandler)
    }

    func stopRecording() {
        movieOutput?.stopRecording()
    }
}

struct RecordButton: View {
    @EnvironmentObject private var cameraManager: CameraManager
    @State private var isRecording = false
    
    var body: some View {
        Button(action: {}) {
            Image(systemName: "circle.fill")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .frame(width: 75, height: 75)
                .foregroundColor(isRecording ? .white : .red)
                .gesture(DragGesture(minimumDistance: 0, coordinateSpace: .local)
                    .onChanged({ _ in
                        if !isRecording {
                            cameraManager.startRecording()
                            isRecording = true
                        }
                    })
                    .onEnded({ _ in
                        isRecording = false
                        cameraManager.stopRecording()
                    })
                )
        }
    }
}

struct VideoPreviewView: UIViewRepresentable {
    @EnvironmentObject var cameraManager: CameraManager

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        let previewLayer = AVCaptureVideoPreviewLayer(session: cameraManager.captureSession)
        previewLayer.frame = view.layer.bounds
        previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
        view.layer.addSublayer(previewLayer)
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {}
}

Another interpretation of that comment is this, which also still has a blank view (but does make the "now" print statement fire).

// ContentView
var body: some View {
    ZStack {
        if cameraManager.isSessionInitialized {
            VideoPreviewView(cameraManager: cameraManager).edgesIgnoringSafeArea(.all)
        } else {
            Text("nope not in this house")
        }
        RecordButton()
    }
    .environmentObject(cameraManager)
}

and

struct VideoPreviewView: UIViewRepresentable {
    let cameraManager: CameraManager
    
    init(cameraManager: CameraManager) {
        self.cameraManager = cameraManager
    }

    func makeUIView(context: Context) -> UIView {
        let view = UIView()
        if cameraManager.isSessionInitialized {
            print("now")
            let previewLayer = AVCaptureVideoPreviewLayer(session: cameraManager.captureSession)
            previewLayer.frame = view.layer.bounds
            previewLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
            view.layer.addSublayer(previewLayer)
        }
        return view
    }

    func updateUIView(_ uiView: UIView, context: Context) {
    }
}

The issue as @vadian pointed out is the bounds were zero since I the view I was taking the bounds from was just initialized. The correct code is a tiny change from my desired modifications. Use UIScreen.main.bounds rather than view.layer.bounds. In the initial code.