I am not a lawyer, and you should talk to a lawyer to get insight on your specific situation.

That being said, in my opinion it is fine to use an existing product/service as inspiration to create your own company. If you can do it better than competitors than congratulations! You will be successful!

The issues you are talking about are usually around similarities in patented tech, products, etc. For example, Apple has a lot of competitors, but there is a reason why none of them seem to build the same product with the same features at the same quality as Apple. They own a lot of the underlying tech. Hope that helps! Let me know if you want to go into more detail.