There is no cost for data transfer from other Microsoft cloud services, i.e. no egress charges between OneDrive and the Cloud PC.
Cost of egress traffic to the Internet is included for the Business SKU for Windows 365 (fair usage applies) - see here: https://www.microsoft.com/en-us/windows-365/faq#coreui-collapsibledrawer-w56p8sc
For the Enterprise SKU it would be charged per GB instead, see here for pricing: https://azure.microsoft.com/en-us/pricing/details/bandwidth/
Accessing other Azure network resources is only possible with the Enterprise SKU. As long as the resources are in or are connected to the same VNET as the Cloud PCs then they can be accessed even if they are not deployed in the same region.
I'm not sure how to answer your second question, it's a bit too general. The Windows 365 service is accessed over the Internet.