Based on @HangarRash's recommendations, I've added the following changes:

DispatchQueue.global().async {
    do { try fileManager.unzipItem(at: sourceURL, to: destinationURL, progress: unzipProgress) }
    catch {  print("error") }
}

Full code:

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    
    let url = URL(string: "1.zip")!
    let downloadManager = DownloadManager()
    downloadManager.identifier = indexPath.row+1
    downloadManager.collectionId = 0
    downloadManager.folderPath = "\(indexPath.row+1)"
    let downloadTaskLocal = downloadManager.activate().downloadTask(with: url)
    downloadTaskLocal.resume()
    
    downloadManager.onSuccess = { [weak self] row in
        guard let self = self else { return }
        DispatchQueue.main.async {
            self.items[row - 1].state = .completed
            self.reloadItem(indexPath: .init(row: row - 1, section: 0))
            
            if fileManager.fileExists(atPath: destination.path){
                let sourceURL = URL(fileURLWithPath: "\(destination.path)/1.zip")
                let destinationURL = URL(fileURLWithPath: destination.path)
                let unzipProgress = Progress()
                let observation = unzipProgress.observe(\.fractionCompleted) { progress, _ in
                    print("Extraction progress: ", progress.fractionCompleted)
                    if progress.fractionCompleted == 1.0 {
                        self.items[row - 1].state = .unzipped
                        self.reloadItem(indexPath: .init(row: row - 1, section: 0))
                    }
                }
                DispatchQueue.global().async {
                    do { try fileManager.unzipItem(at: sourceURL, to: destinationURL, progress: unzipProgress) }
                    catch {  print("error") }
                }
                observation.invalidate()
            } else { print("error") }
        }
    }
    
}