Is this bike right for a 9-year-old?

Likely yes if the child fits a 24-inch wheel bike, which usually suits inseams around 24 to 28 inches. Have them stand over the frame to check clearance before riding.

Does it have a suspension fork?

No. It is a rigid hardtail, so it relies on the tires and rider position to absorb bumps. That keeps it lighter and simpler but rougher on rocky trails.

Are the disc brakes hydraulic or mechanical?

They are mechanical disc brakes, cable-actuated. They stop well in wet conditions but need occasional cable tension and pad alignment checks.

How many gears does it have?

21 speeds, typically a 3x7 setup. That gives a low gear for climbs and a taller gear for flat pavement.

Can it handle real mountain trails?

It handles smooth dirt paths, gravel and easy singletrack. Without suspension and with youth-grade parts, it is not built for jumps or rocky downhill runs.

What should I check after assembly?

Check brake pad clearance, derailleur shifting through all gears, and that the quick-release levers and stem bolts are tight. Recheck spoke tension and tire pressure after the first few rides.