Android is not much of an open ecosystem despite the kernel is [[https://source.android.com/][open source]] due to the most of the phones distributed with the manufacturer's modification.
There are solutions into this such as Android custom ROMs and privacy-oriented mobile operating systems such as [[https://source.android.com/][postmarketOS]] and [[https://lineageos.org/][LineageOS]] but that is not the topic.
However, they are often not ready and/or widely supported.
We may as well take advantage of the Android improvements and features by creating free and open source apps on it.
Here is my personal list of noteworthy Android apps: