Also, I’ve found a way to merge videos without a watermark. But it requires a Windows 10 computer.

You have to use the Photos app. It’s in the taskbar, I guess. Then, select vids you want to merge. Mix the videos together and don’t forget to make a video part longer. The app automatically trimmed a video part into a second. Save it and volla, It’s done! You have to transfer the vid into your phone (unless your PC has a router)