[Edit: if you’re gonna downvote, make an argument]

Real talk. They are not different ideologies. At all.

The reason the words are used interchangeably is because they are, in fact, interchangeable. Any distinction between the two terms is entirely context dependent and one should never assume that anyone you’re talking with shares the same distinction you have for the terms.

We can understand why first linguistically.

Social-ism is the ideology of “social” ownership of the material wealth of society. This is opposed to “private” ownership.

Commun-ism is the ideology of “communal” ownership of the material wealth of society. This is also opposed to “private” ownership.

What is the difference between “social” ownership and “communal” ownership? Nothing. There is no definitional difference between these two words at this level. This is the beginning of the source of your question

We can then understand why they are used interchangeably from a historical perspective.

When Marx and Engels were producing their critique of capitalism and their writings on the type of society of that would come after it, they described that future society as one in which the wealth of society was managed, effectively, as a commons. That means social/communal ownership. At this time, not they nor anyone else in the tradition was making a hard distinction between these terms and they were using them interchangeably.

So they are used interchangeably today for linguistic and historical reasons.

And then we have historical-linguistic reasons. Lenin saw these two terms being used interchangeably and he decided to give them separate definitions. But these definitions were Lenin’s definitions and no one else’s. Some people adopted them, some didn’t, and some adopted them and then later changed their mind. However, it is very important to note that he did not use the terms to distinguish between two different ideologies, he used them to distinguish between two different organizations of society. A communist party, according to Lenin, is a political party that seeks to build communism. There is no such thing a socialist party that means something different than a communist party. But a society is socialist first and then later it becomes communist, despite a continuity of the communist party. Lenin said a socialist society is a capitalist society that is becoming communist and a communist society is one that has achieved communism.

But then the political backlash hit the EuroCentric world (which includes the US). The Nazis were vehemently opposed to communism, but the workers in Germany associated socialism with a movement for a better life. So the German elites made communism the enemy and a taboo, but then the National Socialist party formed. They said “socialism is when workers get what they want” and they promoted better lives for workers to get their support, but they also said “communists are the enemy”. So now we have socialism and communism being framed in a way that is ideologically distinct but in a completely disingenuous and manipulative way.

This sort of perversion continued for a while all over the white world. Communism was “bad” but “unions are socialist” and red scares had to work with the ways in which the communist parties branded themselves as communist instead of socialist. The words kept twisting under the torture of social manipulation in order to obfuscate revolutionary politics.

And now we live in a society where people think socialism and communism are legitimately distinct ideologies, and people believe socialism is fine but communism is just too far, and people believe that communism is a defined phenomenon (moneyless, stateless, classless) with distinct boundaries and a country is either communist or it’s not and that no country has ever been communist.

You are right to ask this question, because you are living a very obfuscated context. But there is no simple answer to your question that is satisfactory. The simplest and most accurate answer is “there is no difference and you can use them interchangeably”. Going beyond that requires engaging with the history and the discourse and it takes a lot of time and effort.