After purchasing you will be redirected to a page that contains your download links.
An email will also be sent to you with a link you can use to access your downloads.
If you used PayPal, keep in mind that the purchase confirmation email + download link may have been sent to your PayPal email address.
The service I use to handle payments and distribution of digital downloads utilizes a self-service system, so if you ever lose your purchase link, you can use the following link to login and access your purchases:
https://pyimagesearch.mykajabi.com/login/
If you run into any issues obtaining your downloads you can of course email me or use my contact form.