As I noted before, this is a (relatively) simple color management issue, because there are a lot more CM options in the newest versions. It's actually easier in the new major version update, 24.0.0, (build 58) than in the 23.x versions. Because all the CM controls can be accessed by one panel ... the Lumetri panel, the Settings tab.
What color space is your media? Newer iPhones seem to default to HDR, but can be set to shoot in SDR (Rec.709) instead. Rec.709 is still easier for most people, and more reliable to get expectable results especially for those newer to video post production.
But no matter whether the clips were shot in HLG/HDR or SDR/Rec.709, they can be worked with on a Rec.709 sequence and export.
Put the clip on a sequence, go to the Color workspace. The Lumetri panel is on the right, go to the Settings tab.
At the top, Preferences/Display Color, make sure "Display color management" is checked, and if on Macs, that the "Extended Dynamic range if available" is also checked.
Typically, it's probably better to then have the Project setting "Auto detect log color space" checked.
In the Sequence settings, set the color space you wish to work in ... easiest, and most viewable on all machines, is still Rec.709. But of course you can choose say HLG if you prefer.
If you set this to Rec.709, make sure that the option below that, "Auto Tone Map Media", is selected. Because in 24.0, if you have "auto tonemapping" clicked on in the Sequence settings, you may not even need to mess with the Source Clip settings. It will normally apply an algorithmic transform to any HDR clip used on the timeline, to very safely get the wider color range and dynamic range remapped to the SDR/Rec.709 color space and range.
In the Sequence Clip section, if you have HLG - HDR clips on a Rec.709 (SDR) sequence, and auto-tonemapping is on, you can also try a couple options to see which you like better. In the dropdown list for "Tonemapping Method are an option called "Hue Preservation (good for iPhone HDR)" that should work, though you might also try with the "Max RGB (good for PQ and HLG)" option.
Then when exporting, you can use the standard SDR/Rec.709 export presets ... easy to see, as they do not have HLG or PQ in the preset name.