144 hours visa free in Shenzhen/Guangzhou

I am a British citizen with UK passport. I plan to visit Hong Kong in June 2024, stay in Hong Kong for a week; travel to Shenzhen and Guangzhou for 5 days, then return back to Hong Kong and stay for another week. (1) Can I apply for a 144 hours visa free for China? How to do that and how must it costs to apply? (2) Do I need to join a tour or independent travel? (3) Can I travel by train to Shenzhen/Guangzhou or I must enter via the airport? (4) Do I need to show the hotel booking prior applying for the 144 hours visa free? Thanks.