In my time, years ago, you had to submit to all schools at the same time. But I notice the kids today can apply to Oxford/Cambridge by mid-October, and then go back to add their other 4 universities as they go along before the January deadline.
The tests and the interview are after application as they are by invite. So being invited to these are signals of success to progress, as some other applicants would be rejected without tests or interview.
If she wants to study PPE, here is a little insight of the top universities a Oxford-quality student might want to consider.
I have arranged them in order of how good they are in the field and in what order she could consider them as her preference positions:
Oxford (1st choice)
LSE (2nd choices)
UCL (2nd choices)
Warwick (2nd choices)
KCL (3rd choices)
St Andrews (3rd choices)
Essex (4th choices)
Durham (4th choices)
Manchester (4th choices)
York (5th choices)
Exeter (5th choices)
Sheffield (5th choices)
Nottingham (5th choices)
SOAS (5th choices)
Obviously the city she would love to study in and enjoy (or not) should play a part in her choice.
Some Cambridge tests are done at interview, but most admissions tests are done in early November (and not by invite).