You shouldn't have to ask someone else to fix a typo.
Editor is the writing surface inside your own console: open a post or a page, type, save. Nothing to install, no second login, no waiting on anyone. You switch it on for a site, and it costs nothing — now or later.
What it writes into
Posts, with Blog
Blog holds the posts and the feed; Editor is how you write them. Draft, revise and publish from the same console — no export, no pasting between tools.
BlogPages, with Site
Site is the site itself — its pages, its look, its address. Editor lets you change what those pages say whenever you like, and read it back before anyone else does.
SiteOne switch in the store, for one site.
Pick Editor in the store and it comes with the site you are working on. Already inside a site? Its settings screen has a panel called What this site carries — and the link at the foot of that panel opens the store carrying this site with it.
Pick one in the store — it will come with this site