There are tons of Notes app available in the playstore and f-droid. I have use my fair share of them these are my best 5 recommendations. All of these are free to use and have to pay extra if you want specific features.
- All in one - Wenote - This is the most powerful note app I have used. This has memo, voice record, calendar, sync, color coding, various fonts, categories etc. Some of these features are behind a paywall. But It is a one time payment. It looks minimal and is light weight.
- All in one but foss - Joplin - This is an open-source project. Available on almost all platforms. If you want a powerful cross-platform note taking application then this is the best bet. This is Completely free but has an option of premium sync option. You can use free sync service to nextcloud and webdav.
- Security - Standard Notes - This is a note taking application that focuses on security. This is an open-source private notes app meaning your notes are end-to-end encrypted, so only you can read your notes. It has a minimal and clean UI. It has dedicated apps for most platforms and syncs your notes securely across all your devices, including your Android devices, Windows, iOS, Linux, and Web.
- Modern - Bundled Notes - This is the most modern looking Notes app on my list. It is aesthetically pleasing and intuitive. A powerful notes, lists, reminders and to-do app. Easily organise notes, lists, photos, files, and more. A google keep alternative.
- For casual use - Notally - A lightweight note taking application. A simple and elegant open source notes app. Notally is a minimalistic note taking app with a beautiful material design and powerful features. Dark mode, Completely free, Adjustable text size, Auto save and backup, No permissions required.
Pretty sure Obsidian is open source.
I wish it was but I think that is the Repo for plug-ins etc. https://obsidian.md/license
Ah - that makes sense