How do I send fiat to Bisq

You don't.

This guide was made as an attempt to garner attention from such questions that get regularly asked on some of our support venues.

Bisq is a DEX (decentralized exchange), it doesn't directly deal with fiat, so it doesn't have builtin fiat accounts, nor credit card payment endpoints; it only manages a BTC wallet (and a special BSQ wallet).

To trade, you need to find another user who agrees with you (selling BTC while you want to buy, or buying BTC while you want to sell, using same currency and same payment method); Bisq will manage the Bitcoin side of the transaction with its battle-tested escrow, and trading peers will manage the fiat/altcoin side on their end, by dealing directly between themselves.

There might be a way to buy BTC with credit card on Bisq though

Technically, you could say you can buy BTC with your credit card on Bisq, even if that's a little more convoluted than what you normally see on e-commerce websites: you would need to setup a payment method that supports receiving deposits from cards, like Revolut, Wise (ex TransferWise), Amazon eGift card to say a few, send money from your credit card to those accounts, and then make, or take, an offer that uses one of those methods.