A company doesn't need a majority of the market to be prosecuted for anti-competitive behavior and likewise having a majority of the market does not mean your every move is illegally anti-competitive.

If prosecuted under the Sherman Anti-Trust Act, existence of a monopoly is a precondition. Certainly there are non-Sherman charges that could be brought, but I'm not aware of anything that would be applicable in this instance?

To your first point, assuming Sherman is used, I believe US v Alcoa [1] is still the controlling case law. And you'd have a very hard time convincing a court that "smartphones made by Apple" constitute a product category over which it could have a 75%+ market share.

Or are there other laws under which action could be brought in this situation?

Yes it is. Pretty much the same case was when Microsoft bundled internet explorer by default in Windows, which was detrimental to Netscape Navigator.

At that time virtually everyone was using windows though. There wasn't a competitor like Android is for iOS.

That’s just incorrect on the face of it. Linux and Mac have existed for as long as Windows, or longer. It’s pure opinion to say there were no competitors.

With monopoly cases, it's all about market share. In the late 90s, Windows had 95%+ of the market[0]. I believe (but may be mistaken) that the legal threshold often used is 75%. Apple isn't there, but Windows certainly was.

[0] https://cdn.arstechnica.net/articles/culture/total-share.med...

since when does market share determine what competitors there are? competitors are businesses competing against each other for the same market share. so your chart actually proves my statement since both linux and apple existed at this time.