They are interchangeable, but I would say that 'start' would be for something quite short term, which will be completed, for example, to start a race, to start watching a film to start preparing dinner and 'begin' for something a bit longer term, begin a project, begin to learn a language, or an activity which may be carried out intermittently, begin to tackle the garden. The use of either would be understood, this is a small distinction.

US speakers use start more often than begin, so unless you want to specifically convey that the thing you are starting is long or not well defined, use start. Using start will also ensure you don't incorrectly say a phrase that requires start: start the car, start from scratch, start off on the wrong foot, etc.

Begin = the first part of an action or process. Start = a point in space or time. Start = cause to happen or begin. This is why they are sometimes interchangeable and why we say "start a/the car" and not begin because as soon as you turn the key or hit/press a switch to start a car or a machine. The process has finished it keeps running until it is stopped or forced to stop. You can start or begin a race, but start is the most natural word to use. You start an enterprise or a business, because this also continues until the business fails or is ended. You can start or begin a journey /trip/ voyage. You would more likely begin a process of testing via many procedures in a scientific setting. Although you could probably use start also.

Both verbs mean the same thing (but as i know begin is more formal than start). And start is used to talk about machines and about creating a new business (we don't use begin in this cases): The engine didn't started. She decided to start a newspaper.

Begin and Start in my opinion are the same