They do have ways to manually add to browsers, apps and search tools.
If you want to manually set Kagi as a default search engine, use these settings:
• Kagi Search URL https://kagi.com/search?q=%s
• Kagi auto suggestions URL https://kagi.com/api/autosuggest?q=%s
They don't have an app.
If you're going to wait to fix climate change, until after you've replaced capitalism first. You might as well work on taraforming Mars and sending billions of people on rockets instead.