what is the difference between the zero spread account and the ecn account?

With Zero Spread Accounts, your broker is the market maker (trading against you) so the broker determines the bid/ask prices and with this arrangement you can get exactly 0.0 pip spread, but you pay a flat commission per trade.

With ECN Accounts, the broker does not determine the bid/ask price instead the liquidity providers (such as banks, hedge funds, oil companies) are the ones that determine the bid/ask price.

The result is that with ECN Accounts, you may not get exactly 0.0 pip spread on all instruments and you also pay a flat commission per trade but the ECN Account commission is always lower than that of Zero Spread Account.

A zero spread account guarantees that you wont pay spread at least on major fx pairs, but it is not a true ecn account & this is an artificial condition which will require you pay higher commissions than on an ecn account which does not necessarily guarantee zero spreads.

I used to trade with Tickmill ecn account and their spreads on eurusd started from around 0.02 pips then there was a $3 per side commission.

During your spare time compare the Tickmill Raw (ECN) Spread Account with Exness Zero Spread Account and you will see the difference in commission payment.

The so called zero spread accounts are offered by market maker brokers to give you zero spread "by design" so it does not come naturally. The ecn accounts give you near-zero spreads naturally depending onthe pricing they get from the big banks who make liquidity possible.