What battery is correct for my phone?

Okay, according to the sources, a model number A2312 battery is made for the second generation iPhone SE, AKA the SE 2020.

With respect to your iPhone 8 comment, well, yes, it is kind of an iPhone 8 in the sense that Apple used the 8 chassis as the basis for an upgraded phone using the newer A13 Bionic processor used in the iPhone 11 models. There are several parts that will interchange between the SE 2020 and the 8, but the battery isn't one of them. iFixit did a comparison between the two phones and worked out which all parts you could interchange, but it turns out the battery uses a different connector and won't work.

Here Are the Parts You Can Swap Between the New iPhone SE and iPhone 8

Also incompatible? The battery. Turns out, although the battery looks identical, the battery’s logic board connector differs from the one in the 8, so they don’t fit together. The SE will connect to an iPhone 11 battery, which uses the same connector—but it won’t turn on. And, sadly, this seemingly throwback phone has some very modern Apple roadblocks inside. You can’t even swap one genuine iPhone SE 2020 battery for another without triggering a “not a genuine Apple battery” service warning.

You can verify which phone you have by looking in the settings or checking the engraving inside the SIM card slot. The SE 2020 will be one of the following.

• A2275
• A2296
• A2298

You can find the model number through the phone's UI by going to the Settings app and navigating to General > About. Tap on the number listed in the Model Number section and it should change to a format that looks like the numbers above.

Assuming you have one of those, then you'll want to stick with a battery with the A2312 part number.

Be aware that Apple is actively working to discourage anyone beside themselves from making repairs on their phones, including even such simple things as changing the battery. So take note that when you change the battery you will get a warning that your battery may not be genuine and you will lose access to the battery's health information. It takes swapping the battery's BMS circuit from the original battery to a replacement cell, then using a device programmer to reset the health information in order to make it work like it did when it was new.