Hi there,
Welcome to visit Shenzhen! Per your questions, hope the following answers will be helpful for you:
(1) For finding the companies
that you’d like to cooperate with, maybe you can send us more details/requirements of the companies, and we might be able to recommend some for you 
(2) Flights:
I’d suggest you flying to Hong Kong since it’d usually be much cheaper to fly to HK than to SZ directly And it’s quite convenient to transfer from Hong Kong to Shenzhen either via ferry or minivan or metro.
(3) Visa:
You can either apply for a visa in the US or get a visa on arrival at the border of Shenzhen. (As a US citizen, you won’t need a visa to get into Hong Kong, but you’ll need a visit to enter Shenzhen).
Visa Type: You can either apply for a tourist visa or a business visa. For business visa, you’ll need an invitation letter from your partner company in China.
Enjoy your trip to China!
Sincerely Yours,
Violet Su
Community Manager @Seeed