If your country of citizenship has a K-ETA program (including US citizens) with the Korean government, you don't need a visa. You only need to apply for a K-ETA visa. This is very easy and quick to obtain. It is valid for 3 years and should be approved in 72 hours. It costs about $10 USD. For additional information, please visit https://www.k-eta.go.kr/portal/apply/index.do
There is an excellent subway system and buses. You will need a public transportation card which can be purchased at select convenience stores in Seoul. You will need a small amount of cash to buy and load it. You shouldn't have issues with exchanging USD to KRW at the airport or other retail banks in Korea.
You can use uber app in Korea. Other on-demand car services may require a Korean cell phone number.
Only if your electronic devices support free voltage. Most modern devices including laptops and cell phone chargers would support free voltage. You will need a Korean outlet adapter. These can be purchased on Amazon and elsewhere. Korea uses 220v not 110v.