Here are main ways to create a slideshow and save it on your iPhone:-
Using the built-in Photos app:-
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Open the Photos app on your iPhone.
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There are two ways to choose your photos:
- From All Photos or Days view: Tap “Select” in the top right corner, then tap each photo you want to include in the slideshow.
- From an Album: Go to the “Albums” tab and open the album containing the photos you want.
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Once you’ve selected your photos, tap the “…” button in the bottom right corner.
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Choose “Slideshow” from the menu that appears.
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The slideshow will start playing automatically. You can customize it while it plays by tapping the screen and selecting “Options”. Here you can change the theme, music, and more.
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To save the slideshow as a video, tap the Share icon (square with an arrow pointing upwards) in the top right corner while the slideshow is playing.
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In the share menu, scroll down and tap “Save Video”. This will convert the slideshow into a video file saved in your Photos library.
More details instruction below.
https://www.youtube.com/watch?v=ZahJZVh0xoQ&ab_channel=TheGeekPage