How do I check this? I cannot find bitcoin.conf and I cannot find a console command that could give me these confirmations.
Do you run a full node on your Windows? I'm not convinced you do. If yes, bitcoin.conf should be in:
C:\Users\<YourUsername>\Appdata\Roaming\Bitcoin
Unless you set another during the installation.
Since I intend to use mainnet only, will it be sufficient to open 8332 mainnet port (in my router port forwarding config.) ONLY?
Yes, although you could avoid port forwarding by just relaying your node's info from Tor. Bisq monitors everything using Tor by default.
[I also see that the default rpc port for regtest for Bitcoin Core is the same as the rpcport, which is a little confusing].
The default RPC port for regtest is 18333
as far as I can see, but you can change it in the configuration file.
[regtest]
rpcport=XXXX
What I’m not understanding is reciprocity, that is, what my running a bisq full node has to do with my use versus the uses of others.
You will run your own node, and connect to it from Bisq, for your own good. It reduces your threat model as it preserves you privacy.