What framework are the mobile apps using? ReactNative? Native?

[CawlinTeffid]

Capacitor.

Why does fs.writeFile not save file?

Only the desktop version of Obsidian uses electron, the mobile version uses capacitor. this.app.vault.create("output.json", jsonContent); would be the correct function call you can omit the options as shown by the ?, they contain stuff that is not necessary here. https://marcus.se.net/obsidian-plugin-docs/api/interfaces/DataWriteOptions