can a usb flash drive be used to run ventura os as a startup disk?

Here's some food for thought regarding external SSDs:

I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties.  Consider one of these depending on your budget and needs: 

1TB OWC Envoy Pro mini - best for portable devices.

1.0TB OWC Envoy Pro FX Thunderbolt 3 + USB-C Portable NVMe SSD

1.0TB OWC Envoy Pro SX Rugged Portable NVMe SSD with Thunderbolt/USB4

1.0TB OWC Envoy Pro Elektron USB-C portable NVMe SSD

Envoy Pro EX with USB-C or USB 3

For your model contact OWC's customer support and get their recommendation for the SSD model that would best suit your iMac model, work needs and budget.  They may have additional models other than those I've shown above.

that is really not recommended as thumb drives are not reliable enough for that purpose. I would recommend getting an external SSD to do that. I have two separate Samsung T7 SSDs that I use for separate macOS boot drives. they work well and are at a good price point.

if you want to use a thumb drive to "just try out Ventura", it likely would work long enough for that purpose. but it might not be fast enough to give you the full experience. but if you like Ventura, then I would at that point get an external SSD.

I have a thumb drive with a different macOS version for the 5 minutes every 6 months that I need to use that OS version. It works but I wouldn't put it under a lot of demand.

Make sure you get a decent quality thumb drive and one that is USB3 rated. I had one that I bought for a different purpose that was part of an inexpensive 3 pack and just using it to hold regular data files it takes about 10 minutes to copy a GB or so over my USB3 port. My name brand USB thumb goes much faster.

You are likely to find running macOS on a flash drive to be unbearably slow. Install macOS on an external hard disk drive or (preferably) SSD. It may boot slowly but once it's running it should run well enough for evaluation purposes.

The installer will ask you for the destination so just be sure to designate it correctly.

Do you have a desktop Mac or a laptop?

I also strongly endorse OWC. They sell items of a quality commensurate with your Apple equipment.