How easy is it to buy firearms in the US?

It can range from somewhat easy to downright difficult.  Just depends on the state you are in.

For instance, if you were to buy a firearm in California, you'd have to be fingerprinted, background checked, pass a gun safety test and wait 10 days to pick it up.  And in California, there are many guns you simply cannot buy because they are not "approved" by the state, and others you have to cripple in some way.

And then there are other states where you fill out a form, the background check is done within minutes, and you walk out with your shiny new gun.

In most states it is very easy.   You go to a gun store.  You find a gun you like.   You fill out the background check form and they run the background check.  You pass,  you pay the money,  you get the gun.  All done in less than an hour.    No permit, registration or license required. 

If one were to believe the "media" all one has to do is go to the supermarket and put guns in your cart and pay for them with your groceries. 

It is harder to buy a house.  Other than that there is nothing that is harder to buy than a gun.

Your question is a bit too general because although there are federal gun laws which affects all 50 US states and 5 territories, each state and territory can have their own sets of laws (as long as they do not conflict with the federal laws), And not only just  federal and states, but individual city/municipalities can also have their sets of laws.

The most general answer I can give you is that if you do NOT live in one of the states such California, Massachusetts, or New Jersey, or cities such as New York City and Chicago, buying a gun is straightforward if  you met the age requirement, you can pass the background check, have proof of residency in the state, and proof of your identity. Proof of residency and identity can be met with your driver's license or any state issued ID that has your photograph  and address on it.  The background checks against the same databases use by US law enforcement organizations, so it is very thorough.

Depends on where you are and what you want to buy. 

Somethings are overpriced due to the pandemic some are overpriced due to undeserved hype. Somethings are banned because your local government doesn't want you to have it other things are banned because the US government doesn't follow the constitution. And as always, depending on your view, the law can be a factor on how easy or hard it is to get firearms from certain "sources".

RIGHT NOW IT IS HARD FOR SOME 1 GOVERNMENT NO LIKE:

Way to easy I don’t care if you’re forbidden from owning one or not. The U.K has the best gun laws.

In a free state, you pick out what you want, fill our a From 4473 and wait while the info is verified.  If you pass, you pay for the gun and go on your merry way.  Other states require that you get a purchase permit, pay a poll tax, wait a while, or some combination of the three.  States that impose a waiting period usually make it 10 days.  They assume that if you have a real need for the gun, you will probably be dead by then and it is one less gun in the state.

Depends on where you're buying them from. Certain states have stricter regulations than others with certain firearms. But if you buy from a licensed gun store (Federal Firearms License (FFL)) you must at the very minimum undergo a background check with the National Instant Criminal Background Check System (NICS). The NICS system is operated by the FBI. These checks can take anywhere from 30 seconds to 30 minutes. The form you must fill out is called a 4473 and to lie on this form is a federal offense. 

Generally, you have to be at least 18, cannot have been convicted of a felony or domestic violence charge, cannot have a restraining order on you, cannot have been involuntarily committed to a mental health facility, cannot be a user of illegal drugs, must be a US citizen, a veteran who discharge was honorable and a number of other things I'm probably forgetting. If you want to buy from a private seller it is no more difficult than buying anything else from a private seller. Some states require background checks be completed even with private sales and some states don't. 

All in all its not as easy as most people think.