Hello Truong,
If I understand correctly, you are wondering why the Estimate Cost process does not include an estimate of resource planned for the project?
During the Estimate Activity Resource process (9.2), the type/quantity/characteristics of resources are identified and they are contained in the Resource Requirements as an output (pg 320-321 PMBOK 6)
Then in the Estimate Costs process (7.2), it will use Resource Requirement (under Project Documents) as one of the input to estimate the cost in each work package. (Pg 240 PMBOK 6)
Keep in mind that many processes during planning can be iterative, they may get revisited and updated multiple times as we find out more information about the project.
Hope this helps,
Joe