These are usually called LSVs; the number of these is low but growing because in dense urban areas or planned developments (golf courses, parks, senior communities) they're far more cost effective.
I've tried obsidian so many times and want to love it but the sync situation - either the expensive official one, the approved options for mixed iOS/Windows clients (icloud) and third party tools like obsidain-livesync - were all way too issue-prone for me to ever trust it. I hope they figure this out
Flightly is really popular on iOS, there's not really a comparable android version. Gentler Streak and a lot of fitness app also don't have comparable android versions - most of the examples I can think of are apps that focus (and charge for) good design
There's many iOS only apps that either don't have anything comparable on android or the alternative is just nowhere near as good (a lot of it is more creative-focused stuff)
A lot of people question the "effective" part of effective altruism, and simply saying "we support more effective giving" is not convincing, especially when the most public figurehead of the movement is/was a convicted fraudster.
1) technically? yes, absolutely- apps like explorer or photo viewer should only use public APIs so other companies can make comparable apps on the OS with 90% market share
2) these are all OS utilities, not workplace apps - there's a big difference between Adobe/Microsoft Office/Google bundling their apps where there's a very clear, very powerful disincentive to compete vs something like explorer.
Zulip is more of a Slack-like instant chat system with threading as a first class citizen; CQ2 looks like threads only exist in the context of one "root" document vs a channel in zulip where threads can intermingle.
Open-source projects where all the developers work for one company tend not to do too well when that company dies, especially if there isn't a large ecosystem around the software yet.