I'm moving around online leftist circles for more than a decade. First I wanted to have some unity with tankies, but I had to slowly learn that it's a futile attempt, with the main issue being their idea of unity being total assimilation.

First of all, just like their authoritarian-right counterparts, don't play by the rules, but expect other to do so, except it's only for the libertarian-left (or anyone who's not as authoritarian as Stalin). This leads to them forming online (and from what I've heard, real life) spaces to their liking, usually with the intent to turn what they call "moderates" into "full-fledged Marxist-Leninists". If you don't they will bait others to harass you, usually by finding something in your past. Often they also work in tandem with far-right groups by providing anonymous information to them, and register accounts to their forums (kiwifarms etc) to get help from them. They often managed to even manipulate the discourse around social causes, they singlehandedly managed to remind people that "spook" also used to be an anti-black slur (they got really angry at egoist leftist memes), and part of the reason why some online leftists are sometimes terminally online about loli/shota (around that time, they really wanted to cancel sex-positive leftists en masse, consensual-noncon also got a hit but that wasn't really part of a fandom that needed a "safe space" for right wingers, hence the frequent cooperation with the right).

One of their greatest display of "manipulating the rules" is their constant redefinition of authoritarianism into "the will of the people" rather than "a hierarchial system of power formed around a select number of people, whom must be submitted to by the rest of the people". On some level, authoritarianism is "will of the people", but tankies (likely intentionally) forget that dictators having pet projects and banning things they don't like isn't the same as liberation of the people who often want to free themselves from those strongmen, and not replace them with someone who promises to be kind. This is not their only use of the "redefinition game": they often like to redefine porn as something inherently exploitative, while promising general sexuality will be fine, then they proceed to act in ways people who define porn as "an ungodly act of perversion", but pretend they're doing it in favor of "progress" rather than saving the masses from the eternal burning fire. On paper, "authoritarian social justice" sounds good, but in practice it can be only be done with things that are "concrete". You can stop people from denying the holocaust, you probably could also stop social media accounts calling all LGBTQ+ people pedophiles, but then there's the issue of sexual objectification in media, which will be on very shaky grounds, and should not be put into law. China put a lot of effort into trying to regulate it, but other than tankie puriteens on Twitter, no one likes it, and often gets workarounds.

Sorry for my personal ramble here, even forgot why I was answering your comment, but the TLDR is that the auth-left and auth-right have way more in common than the auth-left and lib-left, which is especially sad since capitalism itself is a form of authority around wealth.