Quillnote is described as 'Take beautiful markdown notes whenever you feel inspired. Place them in notebooks and tag them accordingly. Stay organized by making task lists, set reminders and keep everything in one place by attaching related files' and is an app in the Office & Productivity category. There are more than 100 alternatives to Quillnote for a variety of platforms, including Android, Windows, Mac, Online / Web-based and iPhone. The best alternative is Joplin, which is both free and Open Source. Other great apps like Quillnote are Obsidian (Free Personal), Standard Notes (Freemium, Open Source), Google Keep (Free) and Microsoft OneNote (Freemium).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.