What decides whether visitors see a page, and which version they see: the published date and the active version.
Two separate switches
Most "why can't I see my change" questions come from treating two separate things as one:
Published decides whether visitors see the record. A record with no publish date, or one still in the future, answers "not found".
The active version decides which content they see. Saving a new version does not change it unless you activate that version.
Neither moves the other. Activating a version does not publish the record, and publishing does not choose a version.
The save buttons
Update version saves your changes into the version you are editing.
Save new version stores your changes as a new version and leaves the active one where it was: the site does not change.
Save and activate stores a new version and makes it the active one.
"Save and activate" is not offered when your account cannot publish the record, or when the record goes through an approval workflow. Which button is listed first follows the setting "Activate new revisions by default", which is on unless your administrator changed it.
Saved is not live
"Save new version" keeps the change out of sight until someone activates it. It is the usual reason behind "I saved it and nothing happened".
Previewing
Live Preview shows the page as you edit it, before you save.
To look at a saved version that is not active, open the page's normal address with ?revision= and the version's id, for example /about-us?revision=3424. It works only for people signed in to the admin; anyone else who opens the link sees the page as it is live, so a preview link that reaches the wrong person shows only what is already public. The versions list at the top right of the edit screen also lets you view and compare versions.
Publishing and unpublishing
Publishing sets the record's publish date. Unpublishing clears it: the page answers "not found" again but stays complete in the admin, ready to be published again. That makes unpublishing the safe way to take something down; deleting removes it, and every link to it breaks.
To publish later, set the publish date in the future and the page goes live by itself at that time. Scheduling content covers dates and time zones.