How to screen record a TikTok Video on iPhone

You can use the screen recording feature built in in your iPhone

See this on how to do that

1. Go to Settings > Control Centre, then tap the Add button  next to Screen Recording.
2. Open Control Centre on your iPhone, or on your iPad.
3. Tap the grey Record button , then wait for the three-second countdown.
4. Exit Control Centre to record your screen.
5. To stop recording, tap the Screen Recording button  at the top of your screen, then tap the Stop button . Or open Control Centre and tap the red Record button .

See this link for further reference

Record the screen on your iPhone, iPad or iPod touch – Apple Support (UK)