I get that you can use a viral vector to introduce designer genes to host cells. The question is, how do you introduce the designer gene into the virus itself?

Does it work the same way as on bacteria? Do you somehow separate the viral DNA/RNA from the virus and then somehow reintroduce it? How do you split open the virus, how do you stitch it back up? What chemicals are used, etc?

I'm thinking about starting a biotech company and selling it once it becomes profitable so that I can shitpost on 4ch*n all day.