I want to access my macbook air desktop contents with my i phone.How do I do this?

Provided that your iPhone and your Mac are both signed into your iCloud account you can use the Files app on the iPhone to access the files on your Mac's desktop.

This further requires that you have activated syncing not just for your iCloud Drive but also for your Mac's desktop:

System Preferences > Apple ID > iCloud Drive > Options > Desktop and Documents Folders

The adjacent box must be ticked to include these two locations.

To be more precise: you are accessing the files in iCloud and not your Mac directly. It works the same way as any iCloud syncing service - the content is on iCloud and all participating devices are downloading a copy.