If you must test a mod APK for research, never put it on your main phone.
The genuine Wondershare UniConverter on the Google Play Store is safe. For occasional users (converting 1-2 short videos a week), the free version is actually sufficient. You just have to watch an ad or keep a small watermark.
Cybersecurity firms (Kaspersky, McAfee) consistently report that entertainment-based mod APKs are the #1 vector for mobile malware. When you install a UniConverter mod, you grant it permissions (Storage, Photos, Microphone). A malicious coder can inject a Trojan that: