can I change system files on my android without them being rooted. And does changing the files mean my phone is now rooted ?
Most system files are protected in different ways.
"system files" viewed as "managed by the Android system"
- everybody can read them, but only root1 can modify ("read-only" for all non-root)
- only root1 has access to them at all ("invisible" for all non-root)
So no: without rooting the device, you cannot modify them directly. Nevertheless, some of them can be modified indirectly, using the appropriate "activities" implemented in the Android GUI – which you might know as "Settings screens".
"system files" viewed as "located on the
Here the entire file-system is mounted read-only – and only root can make it writeable.
The second part of your question is answered implicitly: root being a requirement to edit those files would implicate your device is rooted when you're able to modify them. But before drawing wrong conclusions, you should be sure the file you've modified really is a "system file". To find out whether your device is rooted, there are better means: "root checker".
1: sometimes there are dedicated system processes with elevated privileges, able to "trigger a modification" – this is how those GUI elements (in Settings) work.