From a functional perspective:
XD02 or XD03 – Change/Display Customer Centrally (Including Company Code Data)
VD02 or VD03 – Change/Display Customer in Sales and Distribution (Excluding Company Code Data)
From a technical perspective:
SE11 or SE16 – Used for accessing table data
Table Name: KNA1 or KNB1
You can retrieve all customer-related data from these tables.