Credit cards (Visa/Master/UnionPay) work well in most locations. Contactless is important though - not sure all US cards have that yet. The main things that don't take credit cards are very local eateries and taxis (a few taxis do, but not enough to find one reliably).

All buses (but not minibuses - although tourists in general don't use these) now take contactless credit cards as payment, however the MTR (subway) system doesn't. In general it still makes sense to get an Octopus card, but in general these can only be topped up using HK$ cash (US$ cash is not accepted as a payment mechanism in HK any more than HK$ cash is accepted in the US). You can get an Octopus card at any MTR customer service centre (the most obvious place being at the airport). There are several ATMs at HK Airport where you can get HK$ cash. Choose one that is from a major bank (HSBC Bank of China, Hang Seng) and you will not be charged any fees by the HK bank for the withdrawal. You need to check with your US bank what they charge for overseas foreign currency withdrawals.

If you have a recent iPhone then you can also get an Octopus as an app on that phone. Otherwise do not get a physical "Tourist" Octopus unless you want the card as a souvenir - just get a normal "on-loan" Octopus and remember to hand it in at the airport before you leave to get a refund of the deposit and remaining credit.

Uber exists in HK. Strictly it is illegal, but the government very rarely cracks down, and only then on the drivers not the passengers. It tends to be slightly more expensive than just hailing a taxi and paying cash.