A good place to understand the relation and differences between Security center and Sentinel is : https://azure.microsoft.com/en-in/blog/securing-the-hybrid-cloud-with-azure-security-center-and-azure-sentinel/
This 3rd party article also summarizes the differences : https://medium.com/the-cloud-builders-guild/what-is-the-difference-between-azure-security-center-and-azure-sentinel-9d91eb801cd2
Azure security center mainly focus on your Cloud security posture under 2 pillars :
Cloud security posture management (CSPM) - Security Center is available for free to all Azure users. The free experience includes CSPM features such as secure score, detection of security misconfigurations in your Azure machines, asset inventory, and more. Use these CSPM features to strengthen your hybrid cloud posture and track compliance with the built-in policies.
Cloud workload protection (CWP) - Security Center's integrated cloud workload protection platform (CWPP), Azure Defender, brings advanced, intelligent, protection of your Azure and hybrid resources and workloads. Enabling Azure Defender brings a range of additional security features as described on this page.
Azure Defender is required to concentrate on cloud workload protection within Azure Security Center.
Log Analytics Workspace : ASC, Sentinel needs data to analyze , mitigate and investigate. The data is collected using the Log analytics agents which are stored under Log analytics workspace and which is later used by Security center and Sentinel to show/take actions on different entities.