Earlier quoted context omitted.
You can absolutely have your webcam take a circular form. Just apply a mask to your webcam as a filter and use any image you want to make it take that shape.
But you have to first create a such a mask with some image software: https://obsproject.com/wiki/Filters-Guide#image-maskblend This costs a lot of time and generates questions, for example what resolution should my mask image be? OBS doesn't offer even the simplest default shapes like a circle. This is annoying and makes OBS not suited for beginners and simple tasks.
Not to assume you know how to use masks of course, but this information is readily available when googling something like "webcam circle obs"
Resolution wise I just match the input rather than think about it. If my webcam is 1080p, so is my mask.
I would like if you could natively round elements for sure, dunno when I'd use personally it but it could be useful. There are plugins to do this for you out there too but learning masks and other filters is a super useful skill if you're getting into video
Additionally regarding positioning of things from the top level comment I'm pretty sure there's a positioning menu when you right click that lets you chuck things to the top/bottom/right/left etc