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Where did you download Bitcoin Core, and are you sure your computer is clean?
https://bitcoin.org/ I scan my computer daily is malware and antivirus protection (Malwarebytes and Aviria)
The 12 words came from backup for the wallet, Bitcoin Core shown red bar showing unsecured, so clicking this and backing up provided 12 words that I found out is called the seed.
So I can see my BTC and nothing else. When I go to import seed with Electrum from the read only wallet "Seed" is grayed out.
If I go to new/restore and create new standard wallet with seed I have from my wallet I have to enable BIT39 to proceed, on the next screen I chose legacy, yet tried others. When I go through these steps I see no BTC balance at all.
Do as Abdussamad suggested, and post a screenshot of the wallet software you are using... or show us the exact link that you used to download the wallet.
I assume you got it from here: https://bitcoin.org/en/choose-your-wallet
