If you don't have a mnemonic seed, how do you access the wallet on a new device? Which files do I need to copy from my wallet and where can I find them on Windows OS?
You may always generate a mnemonic seed with your opened cli wallet.
You need to copy the three wallet files in your monero directory.
Open your wallet and type "seed" into the wallet command line. That will display the seed for that wallet. That is the easiest way to go.
Alternatively you can just copy these files from your Monero directory and paste in your new Monero directory: