Sharepoint online - limit Permissions/Views

Dean is correct, you can go down a rabbit hole with this type of requirement. :)

In On-Premise, the way we dealt with this was to use 2 lists. The first list was for the user entry, and in the advanced list settings we set the item level permissions to only allow user to see their own entries.

When an item was created, we had a workflow (you could use Flow for this, or a SharePoint Designer workflow) to copy the information into a second list, where permissions were set for just the managers. They would then work their own list. In this list we would store the ID of the item in the first list so we could always match them up.