Purpose of Transit is to provide a hub and spoke architecture to interconnect VPCs.
Global Transit means that the same Transit Architecture works across different cloud providers meaning you dont have to worry about how different clouds work. Same workflow, same features, same troubleshooting, same visualization etc.
Cloud Access refers to things OUTSIDE of public cloud that connect to cloud. For ex, your branch, office, data center, sites, users, AWS Outpost, Azure Stack etc. They connect to Cloud using Transit but the components of Cloud Access typically live outside of the cloud.