But as both apps link to each other a lot and we share a lot of Linear and Notion links through email, slack or other messaging apps they always open in my browser.
🔗BTT to the rescue
I have configured BTT (BetterTouchTool) as my default browser and set up some automation to inspect the URL and launch them in the right browser before falling back to my default browser.
This set-up works pretty well. It does not work within the browser though. So if you're clicking Notion/Linear links from a webpage (already in the browser) this automation does not kick in as it does not pass through the OS link handling.