In a true ECN model the broker does not take the other side. Orders meet external liquidity, the raw spread can occasionally touch zero, and the broker earns a fixed commission per million traded. See raw-spread-account.
The label is loosely used. Many firms advertising ECN actually aggregate a handful of providers rather than running a network, and some still internalise part of the flow. The meaningful test is the published commission, the average spread data, and the execution policy, not the marketing term.
Example: an ECN account charges $3.50 per side per lot. On EUR/USD at 0.2 pips of spread, the all-in round-trip cost is $7 commission plus $2 spread, or $9, versus $12 on a 1.2-pip spread-only account.
Related: stp-broker, raw-spread-account, commission-vs-spread, no-dealing-desk