The studies and examples in America are small because they are only done on a State level with no to little federal funding. You’ll need to look into how Housing First has been put in practice in European countries to get a better idea about how they can function with federal funding and support.
Not everywhere is equal in Europe, while homelessness is lower across the board because Europe overall has better social services available, Finland is far ahead because of it’s implementation of Housing First.
You also need to look at the definition of homelessness. Finland counts first-stage homelessness, known as couch surfing (which leads to car-camping and eventually the late-stage homelessness of living on the streets) in that calculation. The US ignores 1st and 2nd stage and only calculates homelessness with the late-stage. If the US uses the same calculation as Finland or other European countries, our homelessness rate would be even higher.
Drug addiction is a symptom of late-stage homelessness, not a cause. The cause is almost always the private housing market pricing people out of affording even rent. In the US, housing is first and foremost an investment, not a necessity.
Housing is a human right: How Finland is eradicating homelessness - CBC
Here’s how Finland solved its homelessness problem - WEF