Europe -> Shenzhen -> HongKong

I am German and the plan is to go to Shenzhen and continue traveling to HongKong, stay there and go back the same way. How does it work w/ the visa? Can I use the 144hr-free transit (wanna stay in HKG more than 72hrs)... Will I get the HKG-entry permit at Lohu border? Is there anyone who traveled this way and can help? Thank you very much:)