I wanted to start a pre-bonsai juniper of a smaller dwarf size but a larger and thicker trunk, however all the videos I watched including ones through bonsai empire seemed to incline increasing both the trunk of the tree, and the size. All I want is a thick trunk like the Juniper featured in this video without increasing the overall size of the tree. For example, although I am unsure which is why I am making this post, could a possible method be to keep the Juniper in it's desired pot size, trimming the roots and repotting when necessary of course, while letting the foliage of the Juniper grow, or creating a sacrifice branch to gather nutrients? I would love to hear from someone who is more experienced and knows how to solve this problem. If not, thank you for your time!

The growth of the trunk and brabches is depending on the amount foliage on each one of them. The more foliage the more increase in size each year. So the more foliage you keep the faster it will happen.

From that knowledge, during development, leave it to grow more than in refinement. Cut only for light penetration to inner foliage.

Oh yes, bigger pots, lots of sun, fertilizer and water helps a lot too.

You can use a branch to be sacrificed. One branch you let grow freely for 2 or 3 consecutive years.
You can see an example on the blog of Jonas , Bonsai tonight; bonsaitonight.com/2022/03/04/young-pine-work-with-john-eads/