Minecraft Alt Accounts - Where do they come from?

There are people who sell lists of old accounts. 1000 Minecraft accounts for 10 euro.

What I want to know now:

Where did the accounts come from? How does this work?

Where they are from is different. But you usually get such amounts by hacking / clawing badly protected accounts.

You get acocunt lists on the internet that you feed into a program. Each account is checked if it fits on a MC account. If yes, it will be saved. These account lists look something like this:

Email: Password

Only there are 700k accounts but only 15000 MC accounts. The method is called crack.

Cracking.

Inject a database of old web pages, pull out the data and check with the data to see if they are an MC Acc.