Honkai: Star Rail is a free-to-play role-playing game developed by HoYoverse and is considered a continuation of Honkai Impact 3rd and a sequel to APHO (A Post Honkai Odyssey).
If you haven't played Honkai Impact 3rd, you're missing out on a lot of things.
Honkai: Star Rail is an alternate universe, independent of the previous versions of the franchise. Here the player, acting as a Pathfinder (a vessel for Stellaron), will be introduced to new worlds, cultures, and characters, while also moving along the main path, solving mysteries and puzzles.
Despite the presence of already familiar characters (for example, Armor, Zele, or Himeko), their stories are significantly different from the "original" versions. They do not behave "differently", they are completely different personalities that exist exclusively within Honkai: Star Rail, and do not extend beyond it.
Velt and Carol, who are guests of the alternate world and pursue their dreams while in the "alien" universe, will be revealed as the story progresses.
Therefore, do not play Honkai: Star Rail without familiarizing yourself with the first parts of the series. This will only allow you to see some twists and plot twists differently. For the rest, the entire picture will remain the same: the developers will disclose the alternative universe's history thoroughly so that any player may easily join the new title without having to look elsewhere.