an iTunes library of music etc on a flash drive can i transfer it to my ipad air using the C to USB connector

Assuming that your old PC has your iTunes Music Library, you simply need to connect your iPhone (or iPad) to a USB port of the PC - and use iTunes to “synchronise” your music library to your device.

iTunes has the ability to synchronise other material with your PC and some software if installed - such as Microsoft Outlook - but you have complete control over what you synchronise between your iPad/iPhone and PC. To avoid making unwanted changes to data and files on your iPad/iPhone, before synchronising your devices with iTunes, simply be selective about what you wish to synchronise.

More information can be found here.

Download and use iTunes for Windows - Apple Support

Sync your iPhone, iPad, or iPod using your computer - Apple Support

No. If your intention is to access/play music from your library on your iPad, you cannot directly copy the iTunes music library from a flash drive to the iPad.

Music/Media content present in your PC/Mac Music Library must be transferred to the iPad using iTunes (or Finder on later Mac computers); “Paid” material downloaded from Apples iTunes Service can be copied from the original source to your iPad.

The reason for this intentional limitation, deliberately imposed by Apple, is to protect Copyright. In the case of iPadOS/iOS, Digital Rights Management (DRM) prevents direct copying of copyright material between devices is inhibited. This is intended to prevent the creation of illegal copies of potentially copyright material.

The iTunes Application on your PC/Mac maintains an authorised copy your media - whether this material is loaded from a CD, or downloaded from the Apple iTunes Service - and media derived from this library can be transferred to your iThings.