One way of making hollow objects is to use a core for the first stage. Pinatas, for example, can be built with a balloon core. The core is taken out later, leaving you with a hollow form.
There area huge variety of ways of making hollow clay beads including using a baking soda or corn starch core. The process is, essentially, mixing up a batch of water soluble “clay” that you shape into the cores you want. Let them dry, form your polymer clay over it, bake the clay and then wash the core out of the bead holes. It’s an involved process but it makes great, seamless hollow beads in all sorts of shapes.
ChrirpHop goes through the whole process and some of her failed experiments in her tutorial. The play clay recipe works but if you really want to skip it, get some of the cornstarch peanuts and use those with *slightly* moistened fingers. Instant gooey to make cores out of.