Hi. Firstly Reaper cannot record video although it can edit it. So the most common practice is to:
1. Record and mix your song
2. Plan your video (the"pre-production" phase) where you do all you need for shooting such as "storyboard", research locations, get permissions etc. where required, book "talent" etc. write a script if needed and plan the logistics of the shoot.
3. Shoot ("production" phase)
If you need "lip sync" then you will have to rig some sort of "play back" during the shoot. Because (usually) no live sound will be used you can do that either to the whole cast through speakers or "in ear" if only one person. Note that many music videos don't show much if any lip sync. Totally depends on your "script".
4. "Post production".
Editing sound and images, adding titles, special effects etc.
As I said this can be done in Reaper but (I will be murdered!!) I would say look to a dedicated edit package like Hitfilm, Adobe Premiere Elements, Vegas Movie Studio etc. etc.
BTW the last thing you need worry about is the visual quality. See here:
https://www.indiewire.com/2018/03/mo...ts-1201941565/
Now if you really wanted to record a "live" performance then it might be better to record the performance into Reaper (using separate tracks etc. to allow for sweetening etc. afterwards) while filming on the iphone. Then replace the audio afterwards in the edit program.
If that doesn't appeal then have a look here especially around 9.30 when he mentions using an external microphone.
https://www.youtube.com/watch?reload=9&v=Co21Gw7zqNY
That is probably the most important think to make sure the audio is at least half decent. I have never used an iphone for this but many "live" videos are shot with a single mic eg
https://www.youtube.com/watch?v=7mjuQ67YAnI
As to Search terms - some of the words in quotes is one idea but anything on basic Music Video Production will help. This eg covers the whole process;
https://www.youtube.com/watch?v=ZuRO2bpA3o8 (note the lip sync tip at 5.25).
Is that any help?