The best location are none of one. It's interesting to have help classes or application services. I recommend you to read about hexagonal architecture and a bit of DDD, the part of application/domain services that can help a lot. Also a bit of CQRS or CommandBus too.

Here you can read about it, and here some libraries

I think its neither. It is better to have a different service layer for the logic implementations.

The question you ask is related to software architecture and is more philosophical in nature. So I in your place would, probably, edit a question and add architecture, MVC tags.

It is very hard to tell without any information on "data". You need to provide more examples and use cases.