How do I withdraw from an address
You can withdraw cash, denominated in a currency such as Bitcoin, from an account you have as a customer of an official or unofficial financial institution like a bank, currency-exchange or financial-trading broker etc. You can arrange that the withdrawn amount be paid to a Bitcoin "address" you previously obtained from a Bitcoin wallet you already had.
a Bech32 (P2WPKH) 42 Alphanumerical code that I got no clue how to use.
Yes a Bech32 address is a type of Bitcoin address that can have 42 characters.
It is the sort of address that you can get your Bitcoin wallet to create, typically by clicking on a "Receive" button or similar.
If you didn't create that Bech32 Bitcoin address from a wallet you own, prior to arranging the withdrawal of money from the exchange, then the money wasn't paid to you.
fulfilled by one of their partners called BANXA.
I'm not familiar with Shapeshift and Banxa but it appears that Banxa is a kind of payment processor that can receive fiat payment, exchange fiat for Bitcoin and pay that Bitcoin amount to a provided Bitcoin address. I don't know what value is provided by Shapeshift here but that probably doesn't matter.
Banxa's website, worded for potential partner businesses, says
The customer enters the amount they’d wish to purchase, in either fiat or crypto. The customers enters their email address and mobile. By default, the customer’s wallet is set to your business’s wallet.
So it is possible that instead of paying into a normal Bitcoin wallet you previsouly set up, you left the default "wallet" and Banxa paid your money to Shapeshift. If this is true, you probably have a "custodial-wallet" (the least safe kind) where someone else (e.g. Shapeshift) is looking after your money for you and you just have a kind of IOU. As far as the Bitcoin network is concerned that money belongs to Shapeshift not to you. You should be able to log in to Shapeshift and see if you have an account with a non-zero balance.