How to track down a missing BTC transaction

Hey there... I’m pretty much a total noob. A while back, I bought a tiny bit of BTC and kept it in my BTC App. It did pretty well over time, so I decided to transfer it to BitPay for some shopping. I set up a BCH wallet in BitPay, but I actually sent BTC instead of BCH. Now there’s nothing in that BitPay wallet. Did I just lose my BTC forever?

You could try extracting your private keys from the BCH wallet and importing them into a BTC wallet.

BCH has been tricking people into believing it's real Bitcoin for years after it's creation, but they changed the address format years ago. So by default, I'd expect BCH addresses to be incompatible with Bitcoin now. I haven't tried "BitPay wallet", and it is possible they use legacy addresses. Either way, if it's a self-custody wallet, you should be able to extract the keys or recover them from your seed phrase using iancoleman.io/bip39. You should NOT enter your seed phrase in any website, run this offline and wipe the computer afterwards (if there are still funds in your wallet) before using it online again.

Since you're not being detailed, i would recommend you find TXID of your transaction. Afterwards, copy it to https://blockchair.com/ and see whether it happens on BCH or BTC network.

I just installed Bitpay wallet on my mobile to see how it works. Bitpay generates a segwit wallet for bitcoin and it displayes each bitcoin cash address in two formats. Legacy format which starts with 1 and CashAddr format which starts with q. So, it's possible that OP made a bitcoin transaction to the address generated by Bitpay for bitcoin cash in legacy format. Bitpay gives a BIP39 seed phrase for the wallet and it should be possible to recover the fund, but I couldn't generate the same addresses in electrum. Maybe, Bitpay uses a non-standard derivation path.

Did you try the default derivation path for BCH instead of the usual? Chances are that should work.

I just tried that and it worked. To OP: If you mistakenly sent bitcoin to the legacy address bitpay wallet generated for bitcoin cash, here is how you can recover your fund. Install electrum from its official website. Select "Standard wallet" and then "I already have a seed".If you are on dekstop, click on "options" and check "BIP39 seed" and if you are on mobile, select "BIP39" from the drop-down menu. Enter your seed phrase and proceed to the next level.Select "legacy (p2pkh)" as the type of your addresses and change the derivation path to m/44'/145'/0'