you have the choice, when making a circle skirt, between having an elastic waist (in that case, the inner diameter, and the waistband length, would need to be larger, like a little bit larger than your seat, aka the largest diameter around your butt). It would need more fabric, the circles being bigger.

The other solution is the zip one. It would specifically need an invisible zip, because those are made to “finish” a seam line, by allowing to open it (without it, you wouldn’t be able to put the garment on).

And yes, by “seam”, i mean the stitch line between two pieces of fabric to attach them together. A plain/straight stitch is the way to go, it’s what you need 90% of the time, 9% of the rest would be some kind of zigzag stitch needed for example in stretch fabrics.