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They release updated ISOs on their website for new installs. But once you’ve installed it, you just keep updating via the package manager and it will sometimes be just some apps, and at other times a bigger distro update. It’s best to keep updating (default is a daily check) rather than not checking and then only applying updates a month or so later. But it just keeps rolling on with no need to do a separate upgrade installation.

That said, things can get messy after a few years of installing and uninstalling apps, etc. But I’m running my Manjaro a good three plus years on the same install. I just upgrade my hardware.

Manjaro, better said Arch Linux, is a rolling release. As far as I know, there is no ‘stable’ release, so your system is always up to date with pacman.
If you run pacman regularly, you will never get a “bigger” update as you know from Debian, Ubuntu, Fedora, …

No. But I would recommend EndeavourOS as being a lot closer to Arch and a lot more stable.