A bitcoin wallet does not hold one balance. It holds separate coins (UTXOs), each with its own history. When you pay, the wallet picks some of them as inputs.
The problem
Automatic selection may combine a coin bought on a kyc exchange with a coin received privately. Because of the common-input assumption, an observer can now link both histories to one owner. The private coin's history is now tied to your identity.
Bitcoin's answer
Coin control lets you pick the inputs manually. Many wallets, including Bitcoin Core, offer it, often together with labels.
Good habits
- Label coins when they arrive: "exchange", "salary", "private sale".
- Do not mix sources in one transaction unless you accept the link.
- Watch the change: change inherits the history of the inputs. Treat it as part of the same group.
- Avoid tiny leftover coins ("dust"): spending them later can link groups and costs fees.
- Consolidate carefully, preferably when fees are low and only within one group.
Coin control is not a guarantee of privacy, but it stops you from giving links away for free. It is the most important manual habit after fresh addresses.