It isn't easy, and it isn't always possible.  It really depends on how careful the sender is as well as how careful the people and business that the sender interacts with are.

Many people post their bitcoin addresses publicly.  Others interact with people or businesses that post their bitcoin address publicly.  Many also re-use the same address more than once.

If you do any of these, then yes, it improves the chances that we can find out who is the sender.

In this case the sender re-used addresses and interacted with people or businesses that post their bitcoin address publicly.

Bitcoin is not anonymous.  Anyone who told you it is was either lying to you or misinformed.  Every transaction can be tracked from address to address until it leads to an address that is public.  Once you get there, you just have to ask that person who they sent to (or received from), and trace the chain of transactions back the way you came.