I have an application which is designed using Material Design 2. I have not submitted that to Playstore yet. I am now trying to upgrade that application to Material Design 3. AFAIK, the themes etc present in Material Design 3 will not work on below Android 12 devices. So, I am not able to understand how to support Material Design 2(for below Android 12) and Material Design 3(for above Android 12) in my app.
I understand that this must be some basic question but I am not sure how to achieve this. Do I submit different APKs to Playstore with MD2 and MD3 or within code, do I somehow check the version and decide what to apply?
No, you don't need to submit different versions to Google Play. Take a look at migration article from material blog.