will the unsold tokens be burnt?/
Most certainly yes,thats standard practice now
It need to be done so it doesn't come to any misuse. Many do this that way.
There are some projects that do not burn their tokens so I guess they reuse them on further developement?
There are some, but its generally more insecure practice for the project.
The problem is, even if it's used for good purposed it still brings a level of distrust in the community and tjat's hard to shake of.