According to 9 FAM 402.2-4(B)(1),
An alien proceeding to the United States to marry a U.S. citizen petitioner within 90 days of admission is classifiable as a K-1 nonimmigrant under INA 101(a)(15)(K). (See 22 CFR 41.81.) The fiancé(e) of a U.S. citizen or lawful permanent resident (LPR) may, however, be classified as a B-2 visitor if you are satisfied that the fiancé(e) intends to return to a residence abroad soon after the marriage.
Practically speaking, this means that it is fine for your fiancée to come to the US to marry you using her B visa in the spring of 2021, but if the intent is that after the marriage she should remain in the US to live with you permanently, rather than returning to Brazil, then you should apply for a K visa. Indeed, if she were to apply for a B visa at this time and explained to the consular officer that her plan was to use the B visa to marry you and stay in the US permanently without returning to Brazil first, then her visa application would be denied and she would be told that the K visa is appropriate for such activities.
The "above board" options are thus two-fold: that she come to the US on her B visa to marry you, departs, and then applies for an immigrant visa; or, that she apply for a K visa, comes to the US, marries you within 90 days, and then applies for Adjustment of Status. Between these two options, the latter is usually considered more convenient, as it avoids the need to leave the US after the marriage.
The reality is a bit more complicated since technically she can come to the US on a B visa, marry you, and apply for Adjustment of Status as there is precedent for Adjustment of Status being granted under such circumstances, but this is a more complicated topic, there are risks involved, and I don't think it's wise to rely on this unless there is some good reason why you can't apply for a K visa. See, e.g., this page and citations therein if you want to read about more details.
