The item level permission can be set by multiple ways. Such as: 1. By opening list and sharing item to any user manually using GUI. 2. Programmatically using CSOM, SSOM, REST API. 3. We can also develop a custom SharePoint Designer workflow to set the permission on the SharePoint List items.

Item level permission can be set by 1) Manually explicitly giving permission o each list item. 2) By using Rest API 3) By Sharepoint designer workflow running on each item