This is a small example created that demonstrates the use of a custom UICollectionViewFlowLayout to create cells that overlap each other. This demo was created in response to this StackOverflow question: https://stackoverflow.com/q/71784968/1619193

OverlapViewController.swift

//
//  OverlapViewController.swift
//  TestApp
//
//  Created by Shawn Frank on 09/04/2022.
//

import UIKit

class OverlapViewController: UIViewController {
    
    // The collection view
    private var collectionView: UICollectionView!
    
    override func viewDidLoad() {
        
        title = "Overlap Cell"
        view.backgroundColor = .white
        
        configureCollectionView()
    }
    
    private func configureCollectionView()
    {
        collectionView = UICollectionView(frame: CGRect.zero,
                                          collectionViewLayout: createLayout())
        
        collectionView.backgroundColor = .white
        
        collectionView.register(UICollectionViewCell.self,
                                forCellWithReuseIdentifier: "cell")
        
        collectionView.dataSource = self
        
        view.addSubview(collectionView)
        
        // Auto layout config to pin collection view to the edges of the view
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        
        collectionView.leadingAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor,
                        constant: 0).isActive = true
        
        collectionView.topAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.topAnchor,
                        constant: 0).isActive = true
        
        collectionView.trailingAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor,
                        constant: 0).isActive = true
        
        collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
            .isActive = true
    }
    
    private func createLayout() -> UICollectionViewFlowLayout
    {
        let customLayout = OverlapFlowLayout()
        customLayout.itemSize = CGSize(width: 100, height: 100)
        
        return customLayout
    }
}

extension OverlapViewController: UICollectionViewDataSource {
    
    func collectionView(_ collectionView: UICollectionView,
                        numberOfItemsInSection section: Int) -> Int {
        return 20
    }
    
    func collectionView(_ collectionView: UICollectionView,
                        cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell",
                                                      for: indexPath)
        
        cell.backgroundColor = .systemBlue
        
        if indexPath.item == 0 {
            cell.backgroundColor = .red
        }
        
        return cell
    }
    
    
}

fileprivate class OverlapFlowLayout: UICollectionViewFlowLayout {
    
    // Y Offset for the cells after the first
    let initialYOffset: CGFloat = 30
    
    // X Offset the cells after the first
    let xOffset: CGFloat = 20
    
    // Vertical spacing between cells
    let cellSpacing: CGFloat = 10
    
    // Store the updated content height
    var contentHeight: CGFloat = .zero
    
    // Cache layout attributes for the cells
    private var cellLayoutCache: [IndexPath: UICollectionViewLayoutAttributes] = [:]
    
    override func prepare() {
        
        guard let collectionView = collectionView else { return }
        
        // Only calculate if the cache is empty
        guard cellLayoutCache.isEmpty else { return }
        
        let sections = 0 ... collectionView.numberOfSections - 1
        
        // Set the content height as the initial Y offset
        // so that the second cell overlaps the first
        contentHeight = initialYOffset
        
        // Loop through all the sections
        for section in sections {
            
            let itemsInSection = collectionView.numberOfItems(inSection: section)
            
            // Generate valid index paths for all items in the section
            let indexPaths = [Int](0 ... itemsInSection - 1).map {
                IndexPath(item: $0, section: section)
            }
            
            // Loop through all index paths and cache all the layout attributes
            // so it can be reused later
            for indexPath in indexPaths {
                
                let attribute = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                var itemFrame: CGRect
                
                // Check if it is the first item
                if attribute.indexPath.item == 0 {
                    
                    itemFrame = CGRect(x: 0,
                                       y: 0,
                                       width: itemSize.width,
                                       height: itemSize.height)
                    
                    attribute.zIndex = -1
                }
                else {
                    
                    // Use the current content height as the y position
                    // so the items form one column
                    itemFrame = CGRect(x: 20,
                                       y: contentHeight,
                                       width: itemSize.width,
                                       height: itemSize.height)
                    
                    // Update the y offset by the cell height and spacing
                    contentHeight += itemSize.height + cellSpacing
                    
                    attribute.zIndex = 0
                }
                
                // Set the frame for the current item
                attribute.frame = itemFrame
                
                cellLayoutCache[indexPath] = attribute
            }
        }
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        
        // Retrieve the attributes from the cache
        return cellLayoutCache[indexPath]
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        
        // Get the attributes that fall in the current view port
        let itemAttributes
            = cellLayoutCache.values.filter { rect.intersects($0.frame) }
        
        return itemAttributes
    }
    
    override var collectionViewContentSize: CGSize {
        guard let collectionView = collectionView else { return .zero }
        
        // Update the content size with the new height
        return CGSize(width: collectionView.bounds.width,
                      height: contentHeight)
    }
}