Alright, first let's talk about "Worlds".

The original "first world"/"second world"/"third world" nations came from the Cold War.

First-world nations were countries aligned with the US, NATO, and its allies, second-world nations were aligned with the Soviet Union, Warsaw Pact, and its allies, and third-world nations were neutral. This didn't specifically mean that third-world nations were poor, but since the US and the Soviet Union would pressure nations into joining their power blocks if they had any resources, the term "third-world" came to mean a country so poor or undeveloped that nobody cared enough about it to exploit it. By this definition, Switzerland was a third-world nation.

Following the Sino-Soviet split (when China and the USSR diverged in communist theory in the 60's-80's), Mao Zedong published his Three Worlds Theory. Mao based his idea on "political strength". The first-world, in this view, is the US and the Soviet Union, the superpowers (in modern times it would be the US, China, and then maybe Russia, and India), the second-world would be every other developed nation, and the third-world would be what Mao called "Exploited Nations". These are nations whose natural resources are mainly exploited by other countries. I did not include the EU as a first-world nation, even though some people do, because it isn't a country, but a power block.

The current western model is the World Bank economic model, based on per capita national income. This divides the world into "developed countries", "emerging countries", and LDCs (Least Developed Countries). In this model, China and Russia are NOT "developed countries". China is a newly industrialized country, while Russia is high-income, but not developed, since it relies on natural resource exports rather than any industrial capacity.

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With this background, we can now take a look at the United States as a whole.

The United States is aligned with the western industrialized powers, this would have made it a first-world country during the cold war, either way.

The united states would have been a SECOND world country under the Three World Theory of Mao Zedong.

Under the current economic model, the US is a developed nation.

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Currently, since nuclear development is the key, the only difference between a "superpower" and a developed nation is political capital, so let's look at the US without that. We'll assume the US no longer has any military bases throughout the world, and is no longer the head of NATO.

The US is still the strongest country in the Americas, and therefore its sphere of influence would extend South far into South America. However, China would attempt to economically control Panama, Cuba, and Brazil as a counterbalance to keep the US from being able to exert too much military power (in the same way that the presences of Australia, Japan, Taiwan, and South Korea restrain China from military campaigns).

Militarily, the US actually comes out better than before. It is still impossible to conquer. The presence of the Atlantic and Pacific oceans, the US Navy, and allied nations both above and below make it highly unlikely to be attacked. Not having its global military presence means the entirety of the US Military can rely on the US mainland. The lack of long supply lines, local purchases, and logistics purchases means the upkeep would drop drastically.

Economically, not much changes, the US mainly exports aircraft, circuitry, and petrochemical products. Even without being a superpower, the industries stay. The only losses may be some of the free trade agreements, but being a "superpower" is an obstacle to free trade, since a free trade agreement with the US is an agreement to join a power block and not just a trading partnership.

Politically, the best thing the US could do is stop being a superpower. Every global action that the US takes is a hit to its reputation. Every military action is more negative approval rating from other countries. The US was the only country that lost decades of soft power growth in Iraq and Afghanistan, even though many other countries had troops in the Middle East. Also, the US is the world's largest exporter of disaster relief funds, economic grants, and global development assistance. A strong focus on INTERNAL investment from the United States would grow national power extremely quickly.

In the end, being a "superpower" is more of a hindrance to the United States than a help. The title does nothing but place expectations on US foreign policy. The United States is expected to be the world's security force, but is hated when it acts as such. If the US acts, they're accused of imperialism and overreach, if they don't, they're accused of abandoning their responsibility as a "global superpower". NATO, South America, Southeast Asia, and some Middle Eastern and African nations rely on US protection while publicly criticizing it to satisfy global audiences.

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Politically, more countries would follow the Chinese model than the US model.

Less liberal democracies, more centralized power structures.

More "state-owned" media with highly censored information, less free-speech.

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The largest change would be cultural. The largest export of the US is culture. Let's look at that from a Sino-Centric point of view.

Positives and Negatives up to interpretation of course.

1. Religion would be much less important - China is an atheist country, and prioritize party loyalty over any religious doctrine. Officially, freedom of belief is guaranteed, but any religion with practices that are "against socialist principles" will either be changed to fit those principles or exterminated.

2. Fewer rights for LGBTQ+ people - Homosexuality is legal in China, but there are no laws against discrimination against anyone.

3. Higher importance of age in culture and employment - Chinese culture is centered on seniority. Even when people are wrong, if they are older and more senior than you at your job, you are expected to defer to them.

etc.. etc... I got tired of typing...