large email databases

Hello BHW

I just wanna ask you about email databases I see a lot of people use spam in email marketing

So they get an email database and they send to them a CPA offer

So my question here is

How do those people find large emails (20 million and above) and know their niche and how the sending process go

I have searched a lot about this topic and I did not find any answer

I found Just email marketing with a landing page and so gone