Step 1 is they purify viral RNA straight from a sample of the virus.
Step 2 is they reverse transcribe that pure RNA into complementary DNA with a protein called reverse transcriptase. DNA stocks for RNA production are preferred over straight RNA templates since DNA is more chemically stable and easier to reproduce (either synthetically or through a model organism).
Step 3 is to then synthesize mRNA with the DNA as a template in a process called transcription. This RNA polymerase protein is specifically made to read DNA templates and produce the RNA sequence for them. This RNA is then converted into mRNA with a few other chemical modifications afterwards on both ends of the sequence to stop it from being digested in the cell.
Here a pic summarizing this reaction:
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I’m not sure what tricks go into running this sort of production on an industrial level efficiently but those are the basic principles behind RNA production.