Hi Rong, welcome to Outdoorking. You might take a look at this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=31145&page=1

This picture from that thread shows engine end of both the cutter and wheel drive:
[Linked Image]

There are two pulleys on the front of the crankshaft. The front pulley, right at the front of the mower, is driven through a clutch operated by a spring and lever on the right side of the mower. That pulley drives the cutter deck. The rear pulley drives the rear wheels of the mower. Each of those belts drops vertically from its crankshaft pulley then runs around a pair of idler pulleys that turn the belt through 90 degrees so it is running horizontally toward the back of the mower. The cutter deck belt only reaches as far as the cutter deck, under the center of the mower. The wheel drive belt continues as far back as underneath the driver's seat, then runs a full 180 degrees around a large pair of clutch pulleys that change its direction to forward - see picture from the same thread as before:

[Linked Image]

After passing halfway around those clutch pulleys the belt runs forward and around a spring-loaded idler pulley that tensions the belt. There is a ratchet mechanism so that the tensioning pulley can only tighten over time, not loosen. The two clutch pulleys are parts of two clutches: one for forward and one for reverse. A linkage from the pivoted clutch pedal under the driver's right foot engages one or other clutch depending which direction he or she wants to go. The left end of the shaft running through the hubs of the two clutches carries a sprocket for the chain drive to the rear wheels. The amount of pressure the driver applies to the clutch pedal (top of pedal for forward, bottom for reverse) controls the amount of clutch slip and hence the mower's ground speed.

Because the clutches are large, the system is reasonably durable and these mowers have been used successfully for contract mowing in urban areas.

In case you are interested in details, the mower in your picture (which seems in impressively good condition for an old ride-on) has its cutter drive belt missing. Also, in the side view you can see the wheel drive belt running back under the center of the mower, where it runs over the tops of a pair of idler pulleys that raise it to clear the cutter deck's drive pulley when the deck is in its raised position. Just behind those idlers, you can see the wheel drive belt dip down around the clutch pulleys (see area circled in yellow):

[Linked Image]