Hi Ansuha,

After Creating SLA Definition, in the SLA Definition form you will find fields named "Workflow" or "Flow". Create your own Workflow/Flow as per your requirement and mention your created Workflow/Flow in that Fields.

You can also Use OOTB Workflow named "SLA notification and escalation workflow", Which Uses Elapsed Percentage to Notify "Assignee" and "Assignee Manager".

Please let me know, if you have further questions.

Please accept/Mark the solution as Helpful, if it fulfills your requirement.

Thanks

Vamshidhar