I severely cut back on notifications -- down to just a few apps (SMS/messaging, home security, calendar), and it's had a remarkable impact on my phone use.

The other major change I made was to disable badge notifications on all apps. No more little red number telling me how many unread emails I have, etc. This has had a much greater positive impact than I had thought it would -- I don't find myself compulsively drawn into apps by the lure of "new stuff", and have cut down on my screen time considerably.

I think this strategy can backfire on some people: if you think someone might have messaged you or tagged you in any of 5 different apps you might just cycle through actually opening them.

Have you tried any of the "focus" related Chrome or Firefox extensions or apps?

For me, I've actually used the self control app for a similar experience to Focus. https://selfcontrolapp.com/

How can you turn off all notifications except from one person?

On many android phones there's a "do not disturb" option that can be configured to allow calls and messages from stared contacts and nothing else.

Does it also work selectively for e.g. Whatsapp messages?

Keep whatsapp notifications enabled, but then turn them off in the app's settings. Then, for the person(s) you care about, go to their chat and set up a custom notification.

Yeah, but if I use the "don't disturb" option of my phone, will it not mute all notifications from all apps (except phone and sms-messages)?

You can whitelist apps, too

I disabled all social media accounts and now only get notifications when people call or text me.

Incidentally, very few do (siblings, girlfriend, the occasional friend making an appointment).

If you’re on iOS you can configure specific ring tones & notification sounds for individual contacts. Look under settings.

- to be clear: I’ve found it very easy (no experience with android) to very strictly limit who I’m notified by and the personalized notifications really helped.