Problem with creating Compositional Layout (UICollectionView)

Currently you add HeaderItem to the group, that's why it is limited to the width of the group.

You could achieve the desired effect with a boundarySupplementaryItems for a NSCollectionLayoutSection.

To do it you need to update your HeaderItem:

 let headerItem = NSCollectionLayoutBoundarySupplementaryItem(
     layoutSize: NSCollectionLayoutSize(
         widthDimension: .fractionalWidth(1.0),
         heightDimension: .absolute(35)
     ),
     elementKind: "section-header",
     alignment: .top
  )

Add it to layout section:

section.boundarySupplementaryItems = [headerItem]

Register it:

collectionView.register(HeaderRV.self, forSupplementaryViewOfKind: "section-header", withReuseIdentifier: "HeaderRV")

Create HeaderRV class:

class HeaderRV: UICollectionReusableView {
    
    let label = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.addSubview(label)
        label.frame = self.bounds
    }
    
    required init?(coder: NSCoder) { 
        fatalError("Not happening") 
    }
}

Finally you need to add collectionView(_:viewForSupplementaryElementOfKind:at:) in your UICollectionViewDataSource:

func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
    let header = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "HeaderRV", for: indexPath) as! HeaderRV
    
    header.backgroundColor = .red
    header.label.text = "\(indexPath.item)"
    
    return header
}

Full code:

import UIKit

class ViewController: UIViewController {
    
    private let colors: [UIColor] = [.systemTeal, .systemBlue, .systemGray, .systemOrange]
    private let collectionView = UICollectionView(frame: .zero, collectionViewLayout: ViewController.createCompositionalLayout())
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(collectionView)
        
        collectionView.backgroundColor = .systemBackground
        collectionView.frame = CGRect(x: 5, y: 0, width: view.bounds.width - 10, height: view.bounds.height)
        collectionView.dataSource = self
        
        collectionView.register(MyCollectionViewCell.self, forCellWithReuseIdentifier: MyCollectionViewCell.reuseID)
        collectionView.register(HeaderRV.self, forSupplementaryViewOfKind: "section-header", withReuseIdentifier: "HeaderRV")
    }
    
    private static func createCompositionalLayout() -> UICollectionViewCompositionalLayout {
        let headerItem = NSCollectionLayoutBoundarySupplementaryItem(
            layoutSize: NSCollectionLayoutSize(
                widthDimension: .fractionalWidth(1.0),
                heightDimension: .absolute(50)
            ),
            elementKind: "section-header",
            alignment: .top
        )
        headerItem.contentInsets.bottom = 5
                
        let horizontalGroupItem = NSCollectionLayoutItem(
            layoutSize: NSCollectionLayoutSize(
                widthDimension: .fractionalWidth(1.0),
                heightDimension: .fractionalHeight(1.0)
            )
        )
                
        let group = NSCollectionLayoutGroup.horizontal(
            layoutSize: NSCollectionLayoutSize(
                widthDimension: .fractionalWidth(0.65),
                heightDimension: .absolute(135)
            ),
            subitems: [horizontalGroupItem])
                
        let section = NSCollectionLayoutSection(group: group)
        section.interGroupSpacing = 50
        section.boundarySupplementaryItems = [headerItem]
        section.orthogonalScrollingBehavior = .continuous
        
        let layout = UICollectionViewCompositionalLayout(section: section)
        
        return layout
    }
    
}

extension ViewController: UICollectionViewDataSource {
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 4
    }
    
    func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        let header = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "HeaderRV", for: indexPath) as! HeaderRV
        
        header.backgroundColor = .red
        header.label.text = "\(indexPath.item)"
        
        return header
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: MyCollectionViewCell.reuseID, for: indexPath) as? MyCollectionViewCell
        cell?.backgroundColor = colors[indexPath.item]
        cell?.label.text = "\(indexPath.item)"
        
        return cell ?? UICollectionViewCell()
    }
}

//MARK: - CollectionViewCell
class MyCollectionViewCell: UICollectionViewCell {
    static let reuseID = "myCell"
    
    let label = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        contentView.addSubview(label)
        label.frame = contentView.bounds
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

//MARK: - CollectionReusableView
class HeaderRV: UICollectionReusableView {
    
    let label = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.addSubview(label)
        label.frame = CGRect(x: 0, y: 0, width: frame.width, height: frame.height - 5)
    }
    
    required init?(coder: NSCoder) { fatalError("Not happening") }
}