RNA can be produced in vitro (cell free) using RNA polymerase and DNA with the desired sequence. Large scale biotech manufacturers produce RNA synthetically via high-throughput techniques and provide solutions of RNA spike-ins at predetermined concentration.
RNA can be created outside of cells using an enzyme called RNA polymerase and a DNA template containing the desired sequence.  Large-scale manufacturers use advanced methods to create large quantities of synthetic RNA. These companies provide ready-to-use solutions of RNA spike-ins, which are added at specific concentrations.