How is it possible for a single sedimentary rock to contain rock particles animal shells and minerals that crystallized from water?

Because as the process is making the sediments the fossils and other shrub is washed up together then formed into the rock. When the process is making all sediments the sediments have broken up particles which can be fossils and other things.

"Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution."