Not all communism is authoritarian/statist. "Libertarian communism" (sometimes known as anarchy) is socialist while allowing freedom of association and economic self-determinism. Historically, it has been strongly linked to the concept of republicanism and unions.
What part of it is most socialist? (Honestly asking and curious, I'm terribly ignorant in the more niche -isms of potential political-economic organization)
It's socialist in the sense of promoting mutual aid and communalism. The union movement was originally very influenced by these ideas. Unions, co-ops, FOSS projects, and community organizations are good examples of these type of political/economic structures.
If you're interested, Peter Kropotkin wrote a lot of books and pamphlets that are freely available.
