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.

Edit: Realised that it's DAVx5 that I've got installed which syncs it to my Nextcloud, so you'll need that too.

• Joplin
• Tasks
• NextCloud Notes

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.

Joplin may be an option

Nextcloud Tasks?

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

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.

I use carnet off of fdroid

This is not strictly a to-do list app but a Kanban board. It has many of the same features and It work really well. The api is also amazing if you want other apps to work with it (doing some programming yourself tho)

https://kanboard.org/

Vikunja (hosted on tchncs: https://todo.tchncs.de/) is great for tasks but I haven't tried using it for a grocery list.