Upgrading from iphone X to iphone 15

Most “helpful” Youtube videos are junk. Like the ones you have apparently seen (check wikipedia for Sturgeon’s Law). You can transfer data from an iOS device to another iOS device as long as the target device has the same or a later version of iOS. The only transfer you cannot due is from a newer version to an older version.

Follow this guidance→Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

You can use any of the 3 methods described in Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

The phones don't need to be running the same iOS version; the only limitation is that the new phone cannot be running an older iOS version than the old phone (but that would be a rare case).

CORRECTION:The quick start doesn’t say anything about updating to any version. I just read the link that you posted.

According to certain YouTube videos, both phones need have the same software version in order to transfer all data without using iCloud

You either misunderstood this information......or more likely.....it is just wrong.

Keep in mind that certain YouTube videos claim that the earth is flat. The fact that there might be a YouTube video about anything does not mean that it is true.

The quick start required that boths devices must update however it’s a iPhone X with the latest compatible iOS and the iPhone 15 with the latest iOS that’s all you need plus enough iCloud storage and good battery or boths iPhones plugged with the charger and good strength WiFi signal

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

The quick start required that boths devices must update

Not true.