Am buying iPad Pro to replace my MacBook Air. How do I do this?

You can either AirDrop your documents and files to the iPad, use iCloud Drive or use an external drive.

Photos would need to be either uploaded to iCloud Photos then synced down to the iPad, or again transferred directly via Airdrop or onto an external drive you can use with the iPad.

See the following links:

1. Use AirDrop on your Mac - Apple Support
2. Set up and use iCloud Photos - Apple Support
3. Access and manage your files in iCloud Drive - Apple Support
4. Connect external storage devices to iPad - Apple Support

There is no direct data transfer from a Mac to an iPad however. You will need to do the data transfer mostly manually.

For Contacts, Calendar, Notes etc... if using iCloud to store them, you can sue the same account to access them on the iPad in the relevant Apps.

For Email, you will need to add your account directly on the iPad in Settings->Mail->Accounts or use any 3rd Apps of your choosing such as Gmail, Outlook, etc...

You are going to find out rather quickly, that iPadOS is NOT macOS and many things that your Mac can do NO iPad can.

iPadOS still has many limitations that macOS doesn't.

You cannot run macOS apps on iPads/iPadOS and vice/versa.

If you are used to macOS, you will have a serious learnng curve to get used to iPadOS and its many, current, limitations.

Good Luck to You!