you can't get a dense hedge without clipping.
Each point of termination on every little branch and stem will create a multiple of new shoots. Olives and the like are two new shoots, some go from one shoot to 3 new shoots and some from on e to 5 new shoots. So to get a dense hedge, you must increase the amount of foliage. To do that means clipping and shaping. After a few years it becomes like the one in the video.
Clipping those bigger ones is essential to get any effect like you would have in your mind.
Get some shears, and some seccies (decent ones are advisable like the fiskars ones that roll in your hand, so nice to use ) yo might even need some small loppers on more established shrubs for some stems.
Take to it to get an overall block shape and aim for around a 3rd smaller than it is now.
There will be gaps and holes. Work on them later. Get the shape first
Try not to take more than a 3rd off on your Bush Chrissies. Then take out the longer stems left to just above the nodes. The longer the bit the longer the recovery. So a few mill above the nearest node. Any holes need to have the inside clipped into it. Hollowed out a bit more. Then new foliage is stimulated and the hollow area begins to fill. Then as that grows a little, clip back again. and let it fill more, then clip again and then it will almost be filled.
I would do much more but for a novice this is really easy stuff. Get started and you will see how things work and how they recover. Then you will see how the foliage appears after a few weeks. As the years go on you will need to clip less and less. There are hedges in Europe that are reputed to be 1000 years old and they are clipped every year or two. The leaves will get tighter and tighter, smaller and smaller.
The hedge is a work of art and can be addictive for some once you get the hang of it.
I will post more up later that will help with how to clip so branches grow in a particular direction