Hello @Moti Stein
First, before anything else, I want to encourage you to create more specific names for your sprints than the generic "Sprint 1". Jira does not require sprint names to be unique across the instance. Other projects/boards might also use Sprint 1. And all sprint names across the instance are visible to users. If a user tried to do a search for issues in Sprint 1, they would see multiple "Sprint 1" values and might be hard pressed to figure out which one is relevant.
Regarding your questions:
Jira does not automatically fill in the Fix Version field for you when a sprint is completed, since completing a sprint does not necessarily mean you have assigned the changes to a version.
There is not a pre-defined relationship between Sprints and Versions. There does not have to be any direct correlation between sprints and versions, from the perspective of Jira functionality.
You can set the Fix Version field at any time on your issues. You might set it up front, while issues are still in a backlog or during sprint planning, if you do advance planning for what will be in your versions. Or you might do it after the fact, if you don't know until later which changes will be in your version. You can change the values in Fix Version at any time, and note that the field can have more than one value.
If you set the Fix Version field in advance, planning out which changes will be in which version, then you can use the Releases page to look at the progress of completing those changes.
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