Think of your creature's power as an alternate mana source. Think of the crew cost as an ability that can only use that specific mana source. Crew cost of 4. Tap one 4 power creature or two 2 power creatures, or four 1 power creatures.
It's more nuanced but that's the basics.
Yes! That's where it gets nuanced. You can always overcrew but not undercrew. You can get some goofy tap shenanigans going on! Also weirdly enough you can nest your vehicles. Example you crew a vehicle that has a 2 crew cost. If that vehicle has a power of 4 it can then crew another vehicle w/crew 4
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It's more nuanced but that's the basics.
Could I tap a 5 power creature to power something that needs a crew of 4?
Ofc I can just go read the rules lol.