I understand in Ethereum you can trace it because wallet address never changes. But in Bitcoin you can change your address, no? Thanks.
How can Bitcoin transactions be traced if you can change your address?
In Bitcoin you don't change your address. You create new receiving addresses.
This is normally done so that a new address is created for each new transaction with the intention to increase privacy.
However in each transaction there are inputs and outputs and if a transaction has one input and two outputs, if you can infer which output is change, you know that there is some connection between the input address and the output address. You may be able to then trace that connection further back through a series of transactions. This is approximately like tracing possession of a coin passing through a series of people or businesses in a series of transactions.
This becomes more difficult if transactions have much more than one input and much more than two outputs.
Transactions may have certain characteristics such as address-types, size, timing and so on that may make it possible to make inferences. Transactions with differing characteristics could be inferred as being different parties, those with similar characteristics might be potentially associated with a single party of interest.
For example, if you see a large transaction with hundreds of inputs and one high-value output occur every Friday at 19:00 UTC, you might suspect it was a large exchange moving funds into cold storage as part of it's regular operational procedures. The timing might suggest a specific timezone.
We know that organisations such as the FBI perform blockchain analysis to trace illegal activity. The details of their methods are not published, so far as I know.