As per your comment on @TheDemonLord post, you're asking about how a desert about the size of North America could exist and how it would affect nearby biomes. First, it's important to know what type of desert you're talking about. There are two different types of desert I could see your desert being, and each forms differently.
Based of there being a nearby rainforest, your desert could be a subtropical hot desert. These deserts form along the Tropic of Cancer and Tropic of Capricorn, but since tropical rainforests form along the equator, you might need to change their positions or come up with another hand-wavy explanation for why the desert and rainforest are on the same parallel. If, though, your desert is a subtropical hot desert (the Sahara is an example of a subtropical desert), the surrounding land would most likely be tropical grasslands as tropical grasslands typically form around subtropical deserts and tropical rainforests. Your desert could get as big as it is through climate change. Desertification, the process of grassland turning to desert, is caused by deforestation, over-grazing of animals, and the earth heating up. If these processes occurred naturally (or unnaturally depending on when your story is set) then that could be an explanation for how your desert grew to be so large, however you should probably align it more along one of the Tropics so that it has a better chance of existing.
The other option is for your desert to be a rain shadow desert. These deserts form on the leeward slopes of mountain ranges (leeward meaning facing away from winds, so you would need to flip your wind direction). These deserts form when moist air collects of the wind-facing side of a mountain, causing the leeward side to get very dry. Death Valley in the USA is an example of a rain shadow desert. Real rain shadow deserts don't grow to be very big because they eventually reach too far away from the mountain that is preventing them from getting moisture, so you would need another hand-wavy explanation for that. The other side of the mountain, although being wet, would be cool and therefore your rainforest would more likely be temperate than tropical. Rain shadow deserts in the USA are bordered by temperate grasslands and temperate deciduous forests.
Those of the types of deserts I find most likely be yours. Both require a bit of hand-waving because of how big your desert is, but you can choose which one yours is based on what you want the surrounding land to be like and what characteristics you want your desert to have.
Here's some basic info on different types of deserts: https://education.nationalgeographic.org/resource/desert/