I am probably only going to confuse things more, but here is my best explanation.
Volts and Amps are almost meaningless on their own. Watts is power. Power is work, or what you want to make happen. Your Cpap machine is doing work, it requires a certain amount of power to do this. If they recommend a 150 watt inverter, the device likely only consumes ~100 watts. If you have a 300 watt inverter, it will work fine. It just wont be working as hard as it could.
Watts are a function of current, (amps) and voltage, (volts). 10 amps at 100 volts is 1000 watts, (10A x 100V = 1000W). This is always true in DC applications. It is also mostly true with AC, but AC can get a bit complicated when the current and voltage are not in phase. This happens with coils, and capacitors, but lets ignore that.
If your Cpap requires 120 watts to run, then at 120 VAC, it will draw 1 amp. Your inverter can almost be considered a transformer. At the 12 volt side it will draw 10 amps to make the same 120 watts, (12VDC x 10A = 120W). This ignores the power consumed by the inefficiency of the inverter.
The 5VDC is likely provided by DC to DC converter in the inverter. You can think of this as a transformer as well. DC to DC converters used to be expensive, but not so much anymore. This one is $2.27, free delivery:
P4PM DC12V Step Down to 5V 3A 15W Converter Car LED Display Power Supply Module | eBay