Obsidian app on the phone
Make a shortcut on your desktop so that it's easily accessible and there's minimal friction to make a rough note (you can write on your daily note)
Have obsidian synced to your computer, either through their paid service or something like Google drive / etc.

And now what makes it magic for me:

Have a bash script that runs on your computer every once in a while and combines your notes into a single file
Append that file into your LLM of choice (either online or a local one if you have that setup) and make a prompt that goes something like "you're a NotebookLM style assistant with access to my notes, answer from knowledge of my notes unless explicitly asked otherwise..."

Depending on how sophisticated your setup is, you might get the LLM to automatically pick up changes in your notes. I do this at work and it feels like magic.