要实现合并视频并只播放最后一个视频，可以使用AVFoundation框架来完成。下面是一个使用AVFoundation合并视频并只播放最后一个视频的示例代码：

import AVFoundation
import AVKit

func mergeVideosAndPlayLastVideo(videoURLs: [URL]) {
    // 创建一个合并会话
    let composition = AVMutableComposition()
    
    // 创建一个视频轨道
    let compositionVideoTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
    
    // 创建一个音频轨道
    let compositionAudioTrack = composition.addMutableTrack(withMediaType: .audio, preferredTrackID: kCMPersistentTrackID_Invalid)
    
    // 记录最后一个视频的持续时间
    var lastVideoDuration: CMTime = .zero
    
    // 添加每个视频到合并会话中
    for videoURL in videoURLs {
        guard let asset = AVAsset(url: videoURL) else { continue }
        
        // 获取视频轨道
        let assetVideoTrack = asset.tracks(withMediaType: .video).first
        
        // 获取音频轨道
        let assetAudioTrack = asset.tracks(withMediaType: .audio).first
        
        if let videoTrack = assetVideoTrack {
            // 将视频轨道添加到合并会话的视频轨道中
            try? compositionVideoTrack?.insertTimeRange(CMTimeRange(start: .zero, duration: asset.duration), of: videoTrack, at: lastVideoDuration)
            
            // 更新最后一个视频的持续时间
            lastVideoDuration = CMTimeAdd(lastVideoDuration, asset.duration)
        }
        
        if let audioTrack = assetAudioTrack {
            // 将音频轨道添加到合并会话的音频轨道中
            try? compositionAudioTrack?.insertTimeRange(CMTimeRange(start: .zero, duration: asset.duration), of: audioTrack, at: lastVideoDuration)
        }
    }
    
    // 创建一个播放器控制器
    let playerViewController = AVPlayerViewController()
    
    // 创建一个播放器
    let player = AVPlayer(playerItem: AVPlayerItem(asset: composition))
    
    // 设置播放器控制器的播放器
    playerViewController.player = player
    
    // 播放最后一个视频
    player.seek(to: lastVideoDuration)
    
    // 弹出播放器控制器
    UIApplication.shared.keyWindow?.rootViewController?.present(playerViewController, animated: true) {
        player.play()
    }
}

使用上述代码，你可以将要合并的视频URL数组传递给mergeVideosAndPlayLastVideo函数，然后它将合并视频并播放最后一个视频。