Custom Permissions

This is to give access to users for certain apps, processes, configuration etc. that you have configured and the same cannot be controlled by profile or permission set directly.

Steps:

Setup | enter Permission Sets in the Quick Find box | select Permission Sets | Select a permission set, or create one | On the permission set overview page, click Custom Permissions | Click Edit | To enable custom permissions, select them from the Available Custom Permissions list and then click Add | Click Save.

Reference: https://help.salesforce.com/s/articleView?id=sf.custom_perms_overview.htm&type=5

Visual : https://youtu.be/iDXPr_AbRWM