How could this website be built?

Hey guys, I was browsing the web and found a great website for product mockups like t-shirts etc. and I started wondering how mockup websites like this are made.
I found out that this website is coded with reactjs, but I'm very curious to find out how their mockups work - when you upload your image and it automatically fits it in the mockup with the right perspective and blend modes.
I thought you could only do this manually using photoshop.
Is there any library you guys know that you can name?

The website is: https://mediamodifier.com
Mockup example on their website: https://mediamodifier.com/mockup/a4-letterhead-on-wood-background-mockup/195

Hope that you already found your library, but just in case you didn't I'm pretty sure that you will find it here:
https://openbase.com/categories/js/best-react-image-manipulation-libraries