How can i back up iphone apps and their data (apps bought from itunes and downloaded from elsewhere) to my pc?

I’ve got a jailbroken iphone. I’ve got some apps which i have used and have a lot of notes on, some programs bought on itunes. For example note programs, and i really need to back them up to my pc in case something happens to the iphone. How can i do this?
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How to get back Iphone app on itunes if I’m using a different computer?

I recently switched computer and had to authorize another computer for my itunes/iphone account. I noticed that my Iphone apps that I downloaded on the other computer were not present here, how do I transfer it into my now computer? Do I have to rebuy the apps?