The problem is the following:
I have been developing my game on my laptop until now without a keystore set in Unity editor but I was able to upload my apk file to the Play Store. Now I bought another computer and wanted to upload an update but it gets rejected because it is signed differently.
I tried copying my debug.keystore file from my old computer to my new one, but it also gets rejected because it's for debug.
So my question is: Does Unity or Google Dev Console use a default keystore which the Developer Console accepts? And if so, then where can I find it? Would it be better to publish my application again with a new keystore created by me? (It's only in closed beta, so it's not that big of a deal, but I want to know why this is happening.)
There's no way to recover your key. I once faced a similar situation, since then, I've made several copies of my key. Once again, there is no way to recover this.
You need to create new a keystore and make a backup to prevent any issues in the future