Mesh on this grid
You can bring your own 3D work in here. This page says how that works, what it costs, and where the line is, because the line matters more here than in most places.
Geometry you built somewhere else
Most of what you see inworld is built from simple shapes, joined and stretched until they look like something. Mesh is the other way round: you model it in a program like Blender, export it, and bring the finished shape in. That is how almost everything detailed gets made, and it is open to you from your first day here.
Anyone, once they have done one thing
Every account level may upload mesh. There is no tier you have to buy first. The one thing we ask is that you pass a short quiz about intellectual property. Eight questions, retry as often as you like, no fee, and once you have passed it you never see it again.