Despite eSIM support being present in the Q&A section of the Galaxy S21 series, the models for US carriers didn't have the eSIM capability activated. Until Samsung's latest One UI 4.0 with Android 12 updates, that is, which just made eSIM available on your carrier Galaxy S21 Ultra, S21+, or regular S21.
Just like with the S20 models before them, the S21 phones only supported eSIM functionality in theory, it turned out, but it was not active on US carriers. Apple and Google managed to convince Verizon, T-Mobile, AT&T and the like, to allow the embedded SIM option for iPhones and Pixels on their networks, and it's been running strong on their phones for a while now.
US carriers have long balked at the fact that an "eSIM allows you to activate a mobile network plan right on your phone, so you can switch data plans or add an overseas phone number without the physical Nano SIM," as described by Samsung. They were subject to a probe by the DOJ on exactly that same eSIM matter not long ago, but, at least where Samsung is concerned, the issues appear to have been resolved.
The unprecedented Android 12 update speed of the Galaxy S21 models of Verizon and T-Mobile has unlocked the much desired eSIM capability, too. When listing the new Android 12 update features, Google had a dedicated eSIM section, so the goods may actually be baked in Android 12 rather than come from some One UI 4.0-connected generosity on behalf of Samsung. The aptly named Reddit user i_love_the_usa1776 is reporting that his T-Mobile Galaxy S21 with the newest Android 12 update has received a new eSIM option it the Settings app, and Verizon subscribers report the same.
The fun part is that this eSIM feature allows to be associated with any carrier, so your T-Mobile phone can in theory be linked with a Verizon plan, for instance. The eSIM option arrived on the unlocked Galaxy S21 models with the One UI 4.0 beta, but those are currently not updated to the stable build. As to how adding an eSIM plan to your phone works on Galaxies, you can watch Samsung Germany's eSIM support video, where the feature has been available for a while, with the hope that soon a similar one will be out for the US Galaxy S21 models, too.