Ha! Pretty cool.
So, essentially you have a video in the background with an insert of you speaking and interacting in real time. Is that it?
I don't know what formatting requirements that TikTok has. However, you can do a Picture in Picture with iMovie. You would need a camera recording the background and a second camera simultaneously recording you interacting with the background. You end up with two videos. Put the background video into the main iMovie timeline and the video of you speaking in the upper timeline above it. Then you use the Picture in Picture feature to insert your speaking video into the background video, at whatever position and size that you want. When you are done, export the video out to your desktop for uploading into the TikTok website. Not sure that that's what you want, but I think that that's the best that can be done with iMovie.
-- Rich