Flying and Staying in HongKong visiting factory in Dongguan 1 day, easiest Visa need when land

Since you're flying into Hong Kong and going back, the only legal way to do this without a visa is to buy a HKG-CAN-(third region other than HKG)-HKG ticket and use the 72-hour transit. That is not cheap and will usually set you back a couple hundred bucks at least. You're better off getting a 10-year visa for $140.