What are synthetic indices in forex?

They're not a forex product. Synthetic indices are a separate asset class, traded on the same platforms as forex, but priced by an algorithm rather than currency markets.

Do synthetic indices trade during forex market hours only?

No. Synthetic indices trade 24/7, including weekends, unlike forex which closes from Friday evening to Sunday evening.

Are synthetic indices correlated with currency pairs?

No. Synthetic indices have no underlying currency or economy behind them, so there's no real correlation, any resemblance is coincidental.

What drives synthetic index prices if not economic data?

An algorithm following a pre-set volatility profile, not interest rates, inflation or any other economic data.

How does liquidity compare between forex and synthetic indices?

Forex is the world's largest, most liquid financial market. Synthetic indices don't have a comparable liquidity concept, there's no real order book, the price comes from an algorithm regardless of how many people are trading.

What's the main thing to weigh up when choosing between forex and synthetic indices?

Whether you want your trading tied to real-world economic events (forex) or a consistent volatility pattern with no news to track and no session hours to work around (synthetic indices). Many traders use both rather than picking one.