I've been using Tasks.org which syncs with my Nextcloud perfectly. Had tried quite a few before settling on this as the best option.
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I use Tasks for to dos. Joplin is great for notes, and I use that too for notes.
Orgzly app for android. It is based on org-mode plain text files and support sync via WebDav. Org-mode is one of the oldest format of todo files and have a huge community/plugins/guides/etc.
Because it is plain text in very popular format it may be easily migrated between different cloud storages and may be edited from PC tooo in a any text editor.
Also format of org files is human readable.
Nextcloud Tasks?
Nextcloud is nice since iPhone natively supports caldav/carddav so you can use the built in reminders app.
Joplin may be an option
I love joplin, its so simple and intuitive. I also use it to produce simple manuals and documentation for my code.
It’s the best option I’ve found. Plus the fact that it’s easy to synchronize between iOS, windows and Linux is amazing.
I don't know what features you're looking for but I use nextcloud notes. It's really great for my uses, I keep categorized shopping lists, to do lists, email drafts, etc all in one app. I can access it trough the Android app on my phone and through the web frontend on my nextcloud instance from anywhere.
Vikunja (hosted on tchncs: https://todo.tchncs.de/) is great for tasks but I haven't tried using it for a grocery list.
Vikunja is very slick!
I use carnet off of fdroid
I don't think it's open source but we use our groceries. Just links to an email only no password. They just send you a code when you link the email to a new app. It's also cross platform if that matters at all. Also requires no permissions granted to run and only requests a camera permission to scan barcodes.