How to find my Craft ID account details
You don't automatically get a Craft ID account just from making a purchase.

You'd need to manually create one at https://id.craftcms.com.

If you create one with the same email address you've made purchases with, those purchases and license.key files will automatically be tied to your account so you can manage them and the domains they are associated with.

If you make purchases with different email addresses, you'll have ways of claiming those licenses via alternative email addresses from within your Craft ID account.

If you already have a Craft ID account and you don't remember the password, you can go through the "Forgot your password" link on https://id.craftcms.com by entering your existing username or email.