They say you must send a query letter to the agents.
To make it work well, you must make sure the agent is appropriate to your genre and your book. Will they be an excited fit? Will they be the kind of agent that works well with you and will they be able to promote your book to the publishers?
Doing that takes hours and hours of research. And there are a lot of rabbit holes to follow as you go from agency to agency. I have spent many hours and days doing the research. I hope it makes sense in the end.