Requesting some After Effects assistance

HI!!! I am self-educating myself with After Effects.
I downloaded THIS template because it does most of the concept in my head has thought of.
I've already made some changes, but I'm trying to figure out how to do a couple more things.

1. Increase the view time of each circle. Each circle seems to hover around 1 second. I would prefer it to hover for about 3-6 seconds depending on the clip I use.
2. Replace a picture with a video clip. Instead of a picture inside the circle, I would like to insert a different video clip in each circle.
3. Increase the circles to about 5 or 6 not including the final circle which I plan on being a photo.
4. Increase to 1440p. I changed the composition size to 2560x1440 which seemed to scale other elements which seemed to not really affect the quality much. If there's a better way, please do tell.

I have already replaced the static background and created an animated one so the two Overlay's and White Solid layer aren't needed. The Null 1, I se, controls the rotation. I'm haven't quite figured out how the Camera layer works. The only change in that layer is the Focus Distance. And I'm not quite sure what the third number in the Camera Control layer does under Position. Even if I don't need to mess with either values, I'd like to figure out what each does and how to further utilize them in case I wanted to tweak something later on. Right now it's at 12 seconds, but I think with the added circles it'd be closer to around 25 seconds +/- 5 seconds.

If someone would walk me through the process or create your own template with a video placeholder example so I can reverse engineer it and apply the changes to mine (or reverse), that'd be greatly appreciated.