can a flash drive be used on iPad and iPhone 12?

Yes, a USB flash/thumb drive can be used with an iPhone and/or iPad - with an appropriate adapter.

For devices with a Lightning port, you will require an Apple Lightning to USB 3 Camera Adapter:

https://store.apple.com/xc/product/MK0W2ZM/A

Don’t be confused by the name of this Adapter - as it is the key to connecting many USB devices to your iPad. The adapter incorporates the essential USB Controller/Root Hub that interfaces USB accessories to your iPad’s Lightning port.

You should also note that an iPhone usually cannot supply sufficient power for the drive, so you'll need to connect a Power Adapter rated 12W or greater to the Lightning port of the Lightning to USB3 Camera Adapter

The iPad10 has a USB-C port. Unless the USB thumb/flash drive also has a USB-C connector, you'll still need an adapter. A versatile choice would be the Apple USB-C Digital AV Multiport Adapter:

https://store.apple.com/xc/product/MUF82ZM/A

This adapter similarly supports simultaneous connection of both the USB storage device to the USB Type-A port - and your iPad's Power Adapter to the USB-C pass-through power port of the Adapter. You also have benefit of an HDMI port with which to connect to an external display.

More information can be found in the iPad User Guide - along with other frequently overlooked information about your iPad and iPadOS:

Connect external storage devices to iPad - Apple Support

NB: To be recognised by iOS/iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single partition. Microsoft NTFS and other proprietary device formatting are not supported. Formatting and partitioning, if required, must be performed using a separate computer; iPhone/iPad do not provide any facilities to format an external storage device.