Like any teacher, base64.guru wants his students to not only use the Base64 converter and copy ready-made examples for developers, but also to understand how it works. Since you have opened this page, you may have decided to learn about Base64, and I hope that here you will find answers to all your questions. Otherwise, please let me know what I missed.
And so, first read the following articles:
Next, check the sitemap. I am sure that there you will find additional useful knowledge. I think at least you’ll be interested in reading about the Base64 Standards and Data URI.