Coming from Ireland the idea of a "Left" in American mainstream politics is laughable. We have actual leftwing politicians in Ireland - that is, socialists - which are currently sitting in parliament (which we call the Dáil). Right now these are of the groups Solidarity - People Before Profit (6 members) and Independents 4 Change (4 members) and several somewhat left leaning independents (at least 5), which together accounts for about 9.5% of the the main house of government.
There is no "Left" in US politics with a real voice. The so-called American Left are actually socially progressive liberals, which to bring into context in Ireland are like the, Social Democrats party (which by the way has only 2 seats) and like some members of Fine Gael, the ruling majority party with almost a third of seats. For example, this government called for and supports a referendum to repeal the 8th amendment to our constitution, which prohibits foetal abortion by way of recognising equal right to life of mother and "the unborn". This is the kind of thing that progressives are likely to support in the US.
What I see in commentary on American politics regarding the left is a therefore a conflation of actual lefties with social progressives in politics. The actual left consists of anarcho-communists (simply anarchists to themselves, but an-caps debate the term) all the way to Marxist / Leninist / Trotskyists politicians, which some of the above mentioned currently sitting Irish politicians are or were.
In America "the Left" has been assigned the mascot of the "anti-fa", who are anti facist anarchists generally, often living and practicing their anarchist ideals living in communal squats, flats or homesteads. In recent years they had a huge moment of attention in the Charlottesville riot and Berkeley.
Much more dryly is the establishment left, for example those in upper trade union positions and workers parties. These would be more typical communists as the Americans know them - bureaucratic, officiating, coming from largely middle-class intellectuals and sympathising with so-called oppressed peoples (e.g. supporting Palestine over Israel), post-colonialism, opposing multi-national corporate capitalism, and so on.
If, by the way, you want to see what a anarchist squat looks like, check out this video. And listen to the lyrics, the sentiments are what you typically find with those who write "All Cops Are Bastards" or ACAB on walls.
This is from a series on socialism, anarchism and violence.