38% is sort of an odd number but I would check a few things first. See if you’ve got Optimized Charging turned on. It can be found under Battery settings in System Preferences. Other than that I would download Coconut Battery and see what it has to say about your battery health. Attach the screenshot of your results to your original answer and we can take a look.
As far as which battery it takes, the battery for this MacBook doesn’t have a specific part number that I know off hand. Most vendors identify it simply as the Battery for the A2338 MacBook Pro or by it’s year designation (M1 2020 or late 2020).
And yes. If you want an OEM Apple battery, they only sell them as a part included in the top case assembly. AASPs and the Apple Store offer a Battery Only price point for top case replacement. Which varies by model. But is about $199 USD usually.