Pwm: hot-ground-speed sensor-pwm signal. That's the setup wiring at the header and the primary connector. Fans 2-? have hot-ground-blank-pwm signal.
What happens is the primary sends its speed back to the mobo. No other fans do. The mobo adjusts the fan speed at the header, so any fan on that header (doesn't matter if it's a 2-way splitter or 10-way hub) gets the same pwm signal as the primary. All fans on that header see the same signal. The signal isn't rated at speeds, but duty cycle. A percentage. If the mobo wants it cooler, it sends a higher % signal. That % is according to the fan. If you have a 2000rpm fan, and get a 60% signal, the fan spins at 1200rpm±. If you have a 1000rpm fan and get a 60% signal, it'll spin at 600rpm.
This is why zone control can be important. Your 3 intakes are probably all the same, 2100rpm 120mm. On a 3way splitter, at 60% signal, all 3 fans will spin at 1260rpm±. If your 140mm exhaust is only a 1200rpm fan, and it's on a hub with the intakes, it will depend on which fan is primary. If the 140mm is primary, all fans will spin according to the mobo's desire. If it's running hot, it'll bump up duty cycle, maybe 80% to get the exhaust going faster. Unfortunately that also bumps the intakes to 80%. That starts getting loud. If the intakes are primary, and the mobo is running cool, it'll cut down the signal, maybe to 40%. That means your only exhaust is also at 40%, or a little over 400rpm. If the fan won't go down to 400rpm, it usually just stops.
Hubs primary purpose is multiple fan control from a single header. Like when you have 4,6,8 fans on a single radiator and want all those fans at the same speed. It's honestly better to keep exhaust zone seperate from intake zone, so fans can spin at different duty cycles according to what the motherboard says is needed. There's a temp sensor for almost all headers, only some are ganged. So the temp sensor for the intake header might be near the bottom-right corner. That spot never gets hot. The sensor for the exhaust header might be top-left, which always gets hot. Put all the fans on a hub and plug it into the intake header, your exhaust will run slow, even when the cpu/gpu is in gaming loads and cooking. Put the hub on the exhaust sensor, and your intakes will never slow down, spinning upto maximum when just surfing windows. Using splitters on both, the motherboard retains control of both, so exhausts can speed up and intakes slow down when temps from the individual sensors say its needed.