When you create a segment, by default it is only visible to yourself. If you want other people to be able to use it, you need to "share" it with them. You can do this in the components menu on the left by right clicking the segment, click "share", and then select the users you want to share it with. You can also click components at the top, go to "segments", and select multiple segments from the list to share at once.
Publishing the segment isn't required unless you want to use it in multiple platforms.
Here's an overview of what a user can do with an Analytics segment:
1. Create a segment but NOT share or publish it - in this case, the segment will only be visible to yourself and you can use this in workspace.
2. Create a segment, share it with users, but NOT publish it - similar to above, the segment will only be visible to yourself and the user you have shared it with. Both you and the shared users will be able to use the segment in workspace.
In both cases above, the segment will not be visible in any other products - Audience Manager, Target, Campaign.
3. Create a segment and publish it - Publishing an Adobe Analytics segment to the Experience Cloud lets you use the segment for marketing activity in Audience Manager and in other activation channels, including Adobe’s Advertising Cloud, Target and Campaign. There are several pre-requisites to doing this as shared here - https://experienceleague.adobe.com/docs/analytics/components/segmentation/segmentation-workflow/seg-...
Hope this clarifies?
There are several prerequisites, kindly go through the below shared doc.
Hope this helps clarify your query.