datafile header freeze

when we put tablespaces in backup mode then I understand that the datafile header freezes why does this happen?

and does the control file header also freeze? after the backup mode is over from where do we get the header(scn) information to write through

2. if we need to take multiple tablesapces hot baclup

should we put all the tablesapces in backup mode at once or can we put that at different time?

Won't there be any data inconsistency(scn inconsistency) if we put different  tablespaces in backup mode at different times(not at once all the tablespaces arre put in backup mode) and then try to clone the db at target side