Upvoted the existing answers for "sock puppet" and "alt," but since I have thoughts that are too long for a comment...
A well-maintained "second online life" on some platform is termed an alt account. The most common connotation of an "alt account" in my experience is when someone has, for example, one account where they post things "safe for prospective employers" and a second, probably private, account where they post pics of their wild weekend or their hot takes on national politics or whatever. Or maybe they have one Reddit account for /r/politics and an alt account for /r/hentai (so that the people who know their political views can't easily discover their tastes in pornography, and vice versa).
Alt accounts seem to be especially common among artists — you have a general-purpose account and then an alt account where you post nothing but art, so that people who like your art can just follow the alt. In that sense, an "alt account" is just a poor man's mechanism to achieve something like "post tags" on a platform that doesn't allow it: instead of having your art-followers subscribe to "posts by @Bella tagged #art", you just have them follow @BellaArt.
A related (slightly ironic) use of "alt account" I've seen is to describe the relationship between a writer and one of their online "characters"; for example, comedian Max Miller might refer to @HalfOnionInABag as an "alt account" under the preceding definition; and @HalfOnionInABag, in character, might ironically refer to Max Miller's real account as the onion's "alt."
What OP is describing — using multiple accounts to gin up the appearance of support for one's own position — is indeed sockpuppetry. The fake account itself is referred to as a sockpuppet — at least that's the spelling I would use. I see that other people in this very thread have already used the spellings sock puppet and sock-puppet; there may be some regional variation. (I'm an American thirtysomething with no experience of actual sock puppets; in fact I had to look up to confirm the difference between a sock puppet and a sock monkey.)
The first Oxford English Dictionary entry [for "sockpuppet" with any similar meaning] was "a person whose actions are controlled by another; a minion", with a citation from U.S. News and World Report, March 27, 2000. [However, the citation had nothing to do with online behavior.]