what if even after upgrading to ios 15 on current device the screen doesnt pop up any option to transfer the data

Both iPhones, old and the new, must be on the same iOS version. It appears that you have updated the new iPhone to iOS 15 to match with your old iPhone. I understand that, now you want to transfer data from your old iPhone to your new iPhone. As you have discovered, the transfer will not start automatically from your old iPhone to new iPhone.

Now that your New iPhone has been updated to iOS version 15.x.x, you will have to reset it back to factory settings. To do this, open Settings ➡️ General ➡️ Transfer or Reset iPhone ➡️ Erase All Contents & Settings: Once this action has been completed on your new iPhone, then you will be led back to the "Hello" screen and then an option to hold the iPhone next to your old iPhone and have the data transferred. More details are here 👉 Use Quick Start to transfer data to a new iPhone, iPad, or iPod touch

Axel F.

If you have already set up the iPhone then the data created during this period will be lost if you use any of the processes given below

iPhone to iPhone, there are three ways,

1. Quick Transfer --> Use QuickStart to transfer data to a new iPhone, iPad or iPod ...
2. Restore from iCloud Backup --> Restore or set up your device from an iCloud backup - Apple ...
3. Restore from iTunes backup --> Restore your iPhone, iPad, or iPod touch in iTunes on PC ...

Instead, try transferring data manually. One by One

1. Messages, Media, Voice Memo, Keychain --> Turn on Messages in iCloud & iCloud Photos, Voice Memo, Keychain, and your content is automatically stored in iCloud and is available on all devices that use the same Apple ID
2. Contacts --> Contacts are not stored on an iPhone nor on SIM, they are synced to  Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other. Use the email account that is in your old iPhone.
3. Notes, Reminders, Calendars --> You may sync all these from any of the email accounts that you have on your old iPhone. they can be synced using  Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other.
4. Any Specific Files --> Upload to Cloud Drives (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud and Use Files app on your new iPhone too.
5. Apps & Settings --> In any way they have to be redownloaded using the App Store. Even you had used any of the 3 processes mentioned earlier. Apart from the Apps following will also not get transferred.
   1. Content from the iTunes and App Stores or PDFs downloaded directly to Apple Books
   2. Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos
   3. Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages
   4. Face ID or Touch ID settings
   5. Apple Pay information and settings
   6. Apple Mail data
   7. Apple Music
6. WhatsApp --> Take a backup of chats and media from within the WhatsApp App. Use the same Phone number as that of your earlier iPhone and activate the account in the new iPhone too. That's it.
7. Shazam Account --> Access your Shazams on all your devices or online - Apple ...