Hello Vikas Patel,
Thank you for your post in this Q&A Forum.
Regarding the first part of the question, APIs you are asking about upload, download session APIs are not part of the any API billing models. Here is the list of APIs metered models;- https://learn.microsoft.com/en-us/graph/teams-licenses.
However, it will fall under the SPO throttle limits. You still need to enable Azure billing.
Regarding the second question, "why upload API is using site domain and download content API is using https://graph.microsoft.com" Please specify which part you are referring.
To Download the contents;-
GET, https://graph.microsoft.com/v1.0/drives/{drive-id}/items/{item-id}/content
To upload or replace the contents:-
PUT https://graph.microsoft.com/v1.0/drives/{drive-id}/items/{item-id}/content
Looking forward to hearing from you.
Thanks