I click a link in WhatsApp, it opens up Safari in iOS on my phone, it plays the video, then at the end it starts playing another video.
I'm using a content blocker, so I presume TikTok does not know anything about my personal characteristics, so I assumed the videos that autoplay are the ones they autoplay by default.
PS I will forever harbor resentment to people around the world for putting up with video players that lack the ability to skip around the video or even see the length of the video.
If someone sent you a link, it will include an identifier of the sender. It usually will say at the top of the screen "[Sender account] is using TikTok! Join now". Logically this informs the suggestions of the app.
The lack of scrubbing is annoying (although apparently the Android version recently added the ability). The app is also inconsistent in that sometimes it shows a progress indicator at the bottom, sometimes it doesn't.