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.

• Joplin
• Tasks
• NextCloud 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?

Joplin may be an option

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.

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.

Google play link