Programming devices in Android I wanted to know that if all the devices that support Android OS can be manually reprogrammed or do they allow to make own apps and load in them, just in case I want to program an app for myself, or it's restricted to specific devices. How shall I know?

Update: For example, I want to program a personal app to prepare notes in handwritten mode using a stylus and integrate other mathematical instruments in it.  Expecting features like adding space in case I want to add notes somewhere at later point of time or completely erase some specific portion of page.
You can create an application for any device that runs the Android OS, this is not restricted to specific device manufacturers.
Restrictions in the execution are caused mainly because of the version of the Android OS that runs on the device (the differtent API's) or the difference in hardware capabilities between devices.

If you are planning to make an application you should by all means have some programming experience. With the use of some very helpful software development tools you could create personal application like suggested. Be sure to check out the documentation provided on Androids official website  to get a solid introduction on this topic.