The thing to understand is that Roll20, D&D Beyond (DDB), and Wizards of the Coast (WotC) are all separate companies. Wizards of the Coast owns the Dungeons & Dragons content, and licenses it to Roll20 and DDB, and each of those licenses is a separate agreement on how to sell & distribute the content to customers. The Essentials Kit was a one-time collaboration between DDB and WotC to give access to customers who bought the physical books on DDB. But otherwise there is no connection between physical purchases made to DDB or Roll20. When you buy a physical copy of the Players Handbook or Dungeon Master's Guide or any other sourcebook, DDB and Roll20 do not get any money from that. With that being said, you are perfectly free to create/copy content that you own in one form for use in another form for personal use. So scanning the books you own and making them into digital versions and uploading into Roll20 is perfectly fine, as long as you're not selling it to others. Taking digital versions from DDB and importing it into Roll20 is also fine. It just takes time and work to do so. I've found that repurchasing adventure modules on Roll20 is worth the time and effort it would take me to recreate all the maps, dynamic lighting lines, tokens, etc. on Roll20. In addition, if you already have content on DDB, but you want to use Roll20 for the VTT, you can use a browser extension called Beyond20 to port rolls from DDB into Roll20. It will not move any content into Roll20 (such as maps or tokens), but it will allow you to use the character sheets on DDB, and your players can click on buttons on DDB, and the rolls will show up in the Roll20 chat window as though they had clicked on character sheets in Roll20. You will still need to create/import/purchase whatever maps and token images you want to use in Roll20.