FOSS software is developed in such a way that you can build it yourself freely (In other words, you can download the source code and compile the actual application yourself, free of charge). Obsidian doesn’t really work that way. Even if most of the code is available, the full app is only available as prepackaged binaries which might introduce god knows what (and make forking the application impossible).

Well, by just searching topics in the search bar you can typically find instances related to the search. You need to click the “chain” icon rather than the “federated star” icon to view the post “from your instance” and stay on your personal account.