The iPhone 12 is a huge upgrade from the iPhone 7. Updated CPU and the camera has better fstop for lighting.
Thanks.  There are different types of iPhone`12. I do not understand the differences.
It depends on what you want to use it for. The iPhone 12pro and max have bigger screens, more storage and a better camera. If you like to take pictures, they are a better choice. If you are working with a budget, the iPhone 12 or iPhone 12 mini are the best option.
How about the differences in prices?  I want to get the best one if I can afford it since I will keep it for a long time.
The top of the line, iPhone 12 Pro starts at $1,099 with 128GB of storage. That's plenty of space for photos and apps. You But your phone service provider might be offering a deal. You should check with them before paying full price.
Good idea.  I will certainly go to T-mobile site to see if they offer their customers good deals on iPhone12.
T-Mobile often has deals where you can trade in your old iPhone to receive a discount on the new iPhone. It often depends on the condition of your trade-in. Check with them if they are offering any trade-in promotions.
What is the difference between Pro and Max?
For $99 more, the Max has a larger screen at a whopping 6.7 inches. It's definitely great for watching movies or taking videos. If you have small hands, it might be difficult to work with. 
I do appreciate your help.  I am old and so Max will be suitable for me. I thank you very much for your time.