[WhiteSage]

It sticks to a virus protein, so you can use that virus protein to find it!

[dukoid]

Is synthetisizing antibodies a thing? Is it hard? If so, why?

[fabian2k]

They're far too large and complex to be synthesized from scratch, that is simply not possible.

They are produced in various biological systems, with nature doing the synthesis. As far as I understand you can scale that up reasonably well with some effort.