Answer 1

Concurrency means executing multiple tasks at the same time but not necessarily simultaneously. In a concurrent application, two tasks can start, run, and complete in overlapping time periods i.e Task-2 can start even before Task-1 gets completed. Any task can have executed for any time period according to their importance for example an father is baking cake for his daughter and at the time of baking he got an important call so first he attend that call for some time then go again for bake cake in this way one more person rang the 🔔 of house then he leave baking and get to handle the person on door this way he do simultaneously task and complete them

concurrency is achieved via a process called context-switching. If its a multi-core environment, concurrency can be achieved through parallelism.
Parallelism

Consider you are given two tasks of cooking and speaking to your friend over the phone. You could do these two things simultaneously. You could cook as well as speak over the phone.

Answer 2

You can get information about multi tasking and multi threading
https://tutorialwing.com/difference-between-multitasking-multithreading-and-multiprocessing/

Mostly all this are used in OS fields