There are a couple of options here. One, which I’ve found the easiest, is to move content you want to protect into custom post types. From there, you create specific permissions (capabilities) for each CPT, and WP will automatically only show the content that the current user has permission to edit in the back end.
The other option is to seek out a plugin. There are a few role plugins available which allow you to set permissions in various ways, such as selecting certain pages that each user or user group can edit. You’re then dependent on the plugin, but it’s easier to set up and get going that way and doesn’t require any URL changes like CPTs may.