I am unable to save 2160x3840 videos into iphone Photos App

Hello susinfo,

If you are having trouble saving long videos on your iPhone, you can try to share them via alternate means as explained here: Share long videos on your iPhone - Apple Support. Review these options:

"Send a video using AirDrop

With AirDrop , you can send a large video to a nearby iPhone, iPad, or Mac. AirDrop sends information using Wi-Fi and Bluetooth, so before sending, make sure both you and the recipient have these controls turned on in Control Center.

Note: AirDrop requires iOS 7, iPadOS 13, OS X 10.10, or later.

1. If the person you’re sending the video to isn’t in your contacts, have them do one of the following:

• On iPhone or iPad: Have them go to Settings  > General > AirDrop, then tap Everyone for 10 Minutes.
• On a Mac: Have them go to Apple menu  > System Settings > General > AirDrop & Handoff, then click Everyone.

1. Open your photo library, tap the video you want to send, then tap .
2. If you want to send the video in its original format, including metadata, location, and any associated edit history or captions, tap Options, then turn on All Photos Data.
3. Tap AirDrop, then tap the contact or device you want to share with.
4. After you share, the recipient receives an alert to accept or decline the AirDrop transfer.

Send a video using an iCloud link

An iCloud link is a URL that you can use to send a large video with Messages or Mail.

To send an iCloud link, make sure you have iCloud Photos turned on. Then follow these steps:

1. Open your photo library, tap the video you want to send, then tap .
2. Tap Options, tap iCloud Link, then tap Done.
3. Tap Messages  or Mail , enter the recipient of the iCloud link, then tap  to send the message or email.
4. Recipients have 30 days to download a video sent with an iCloud link.

Send a video using Mail Drop

With Mail Drop, you can send a large video as an attachment in Mail.

Note: Mail Drop requires iOS 9.2, iPadOS 13, OS X 10.10, or later.

1. Open your photo library, then tap the video you want to send.
2. Tap , then tap Mail.
3. Tap the To field, then enter the name or email of the recipient.
4. If you want to add a subject or message, tap the Subject field or email body.
5. Tap  to send. 
6. Recipients have 30 days to download Mail Drop attachments."

We hope this helps. Let us know if we can assist with anything else at all. Take care.

This method is only from a Photos app where the video was already imported, how can you import it without having the video in another Photos app? (but in Files for example)

Edit: also just tested from MacOS photos app and still doesn't work.

Is there any documentations about the limitations of the iOS Photos app?