"Focus" and "concentrate" are interchangeable in some sentences. But both words have multiple definitions, so when used with their other meanings, then they are not interchangeable.
When the meaning you want is "to cause (attention, efforts, strength, etc.) to be used or directed for a single purpose" then focus and concentrate can be used interchangeably. For example, "She can concentrate (focus) better when she studies in a quiet place." "The city is focusing (concentrating) its efforts on building temporary shelters so that no homeless people will be sleeping on the street."
In my opinion, I think focus can sometimes be a better verb in situations when attention is being given to something but without a goal or effort to try to do something . For example, "During the election, many people focused on the candidate's past romantic relationships rather than whether he had the experience to do the job." I don't think "concentrate" would work as well in that sentence.
Focus and concentrate are not interchangeable for their other meanings, including when they are used as verbs in these ways:
Focus can mean " to adjust (something, such as a lens or a camera) to make an image clear" or "to cause (light) to come together at a point"
Concentrate can mean "to make (something, such as a liquid) stronger by removing water.