Setting aside the motivation for switching entirely from iPad to iPhone (many users have both - as they usefully complement each other), from the perspective of data, there are no specific consequences. Data stored in iCloud is just that - its stored in iCloud - and is not transferred between devices.
Data stored locally on your iPad can be transferred to a different device - such as an iPhone.
These support pages should get you started with setup of a new iPhone or iPad - and the methods of transferring your data from one device to the other:
Set up your iPhone, iPad or iPod touch – Apple Support
Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support
Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support
If the donor device, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.
“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.
An alternative is to restore either an iCloud or iTunes backup to the new device:
How to back up your iPhone, iPad and iPod touch – Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Apps themselves are not directly transferred between devices. For Apps, the transfer process creates a record of the Apps that are installed on the donor device - and it is this record of installed Apps that is transferred to the target (new) device. For Apps that are still available for download from the App Store - and are compatible- the corresponding Apps are automatically downloaded and installed from the App Store.
Not all Apps will, necessarily, continue to be available. Apps may have been removed or retired from the App Store by the respective App Developer. Some Apps may have been abandoned - and may not be compatible with the new device. Where a compatible App is not available, you’ll not be able to transfer/install the App.