PyQt6 : Use python with ffmpeg tool ...

If you download a video from youtube with high resolution using a tool as yt-dlp then you can get two files with video and audio content:

... one with be with .f401.mp4 and another with .f251-9.webm and using the ffmpeg tool you can create one .mp4 file with both audio and video content.

Let's see a source code with python and PyQt6 module to search into D:\Software folder and create the mp4 file.

import os
import subprocess
from PyQt6.QtWidgets import (
    QApplication, QWidget, QVBoxLayout, QPushButton,
    QListWidget, QMessageBox
)
# fisier download : yt-dlp.exe -vU https://www.youtube.com/watch?v=xxxxxx -f bestvideo*+bestaudio/best
FOLDER_PATH = r"D:\Software"

class FFmpegMerger(QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("Combinare Video + Audio cu FFmpeg")
        self.resize(600, 400)

        self.layout = QVBoxLayout()
        self.file_list = QListWidget()
        self.process_button = QPushButton("Prelucrează în MP4")

        self.layout.addWidget(self.file_list)
        self.layout.addWidget(self.process_button)
        self.setLayout(self.layout)

        self.process_button.clicked.connect(self.process_files)

        self.populate_file_list()

    def populate_file_list(self):
        files = os.listdir(FOLDER_PATH)
        video_files = [f for f in files if f.endswith(".f401.mp4")]
        audio_files = [f for f in files if f.endswith(".f251-9.webm")]

        base_names = set(f.split(".f401.mp4")[0] for f in video_files)
        candidates = []

        for base in base_names:
            audio_name = f"{base}.f251-9.webm"
            output_name = f"{base}.mp4"
            if audio_name in audio_files and output_name not in files:
                candidates.append(base)

        for name in candidates:
            self.file_list.addItem(name)

    def process_files(self):
        for i in range(self.file_list.count()):
            base = self.file_list.item(i).text()
            video_path = os.path.join(FOLDER_PATH, f"{base}.f401.mp4")
            audio_path = os.path.join(FOLDER_PATH, f"{base}.f251-9.webm")
            output_path = os.path.join(FOLDER_PATH, f"{base}.mp4")

            cmd = [
                "ffmpeg",
                "-i", video_path,
                "-i", audio_path,
                "-c:v", "copy",
                "-c:a", "aac",
                "-strict", "experimental",
                output_path
            ]

            try:
                subprocess.run(cmd, check=True)
            except subprocess.CalledProcessError as e:
                QMessageBox.critical(self, "Eroare", f"Eroare la procesarea {base}: {e}")
                return

        QMessageBox.information(self, "Succes", "Toate fișierele au fost prelucrate cu succes!")

if __name__ == "__main__":
    app = QApplication([])
    window = FFmpegMerger()
    window.show()
    app.exec()