iphone external display

Yes, see the tech specs.

iPhone 15 Pro and 15 Pro Max - Technical Specifications - Apple

Video Playback

• Supported formats include HEVC, H.264, and ProRes
• HDR with Dolby Vision, HDR10, and HLG
• Up to 4K HDR AirPlay for mirroring, photos, and video out to Apple TV (2nd generation or later) or AirPlay‑enabled smart TV
• Video mirroring and video out support: Up to 4K HDR through native DisplayPort output over USB-C or USB-C Digital AV Adapter (model A2119; adapter sold separately)13

Who said anything about mirroring photos? Mirroring has always mirrored the entire screen with whatever app is opened, which would include Excel.

I don't see why the mirroring feature would be less advanced in a new iPhone than the old ones and current iPads that work the exact same way.

That’s not an announced feature of the phone as far as i know. i wouldn’t think so.

Mirroring photos and videos is a long way from being able to use the external display for apps like Excel.