It’s a bit unclear what you mean by “Apple” - I’m assuming you mean Safari on both Mac and iOS.

The search engine I use is SearxNg. On Firefox on Mac it was pretty easy to add.

To use it in Safari, I installed the Keyword Search extension from the App Store. It has the option to set a search engine as the default if you don’t use a keyword, so I did that. This works in both Mac and on iOS / iPadOS.

There are other Safari extensions that do similar things, like Customize Search Engine (free). Kagi has an extension that can make Kagi the default search engine, for example (it doesn’t appear that there’s an equivalent for Startpage, though). I haven’t used anything other than Keyword Search for this, though.