How do you write marketing copy for tech companies, especially startups, if you don't have any technical knowledge about their product or service? Is it just possible with research and experience?

Like any marketing copy, you research.

find the ideal prospect

find out what problem or issue the product solves and how

learn the technical aspects that are important to the ideal prospect

learn the ideal prospects view of life, problems, challenges, solutions, etc

Research is usually half of all the time spent by a good copywriter.