Beat the game then buy it for two million Pesetas
Esoteric unlock requirements are back in the Resident Evil 4 remake, with various rankings (read: grades like A, or S) gating certain upgrades and extras. But the infinite rocket launcher is up for grabs after just one playthrough: provided that you have the cash to spend on it.
After finishing the game, find the Merchant in your NG+ run to buy the Infinite Rocket Launcher in the Resident Evil 4 remake
Funnily enough, one of the most famous weapons in Resident Evil history is also one of the easiest to unlock: technically. But you’ll need to sell a ton of treasures to actually be able to afford it.
After going through the initial village segment in the remake and encountering the first Merchant shop, you’ll be able to purchase the Infinite Rocket Launcher for 2 million Pesetas (2,000,000 gold). It’s a massive sum of cash, but given that you can sell some treasures for roughly 100,000 Pesetas, it’s something you can work toward: especially if you’re doing your NG+ run on an easier difficulty than Professional.
Remember, The Merchant first appears in the game at the start of Chapter 2; right after the short sequence that involves getting your gear back. This is the soonest you can buy the infinite rocket launcher, but The Merchant will carry it at every single spot in the game. If you need to come up with the cash eventually, he’ll be waiting for you.
