We've just launched on Android. What would be the best way to attract a large number of users?

Of course, totally depends on the type of app, and the target audience. But generally, social media and ads on social media would be my first thought (LinkedIn, IG, Facebook, Twitter, Reddit, Pinterest, Quora). Find communities there that have the target audience, and promote your app there. Consider collaborating with influencers whose branding matches the app and can reach the people you're targeting. I downloaded several apps (both for professional use, for hobbies, and for fun) after seeing an ad on IG or hearing a person describe the value that they got from using the app.