How can I download Lightroom app on my iPhone?

The first thing you need to have is an iPhone which can run iOS 16 or higher. Assuming you meet the minimum requirements, here is a link to Adobe Lightroom App --> https://apps.apple.com/us/app/lightroom-photo-video-editor/id878783582

Assuming it’s available in your region and you have a device it is compatible with (currently requires at least ios 16), you download it from the App Store just like any other app.