What is exactly the difference between sellside and buyside?

I have done some research on my own and skimmed through Rosenbaum on my own but I am hoping to get some insights to have a more firm grasp behind sell-side and buy-side deal processes in general. I understand how banks typically provide these services to clients that they have previously worked with, but if they want to generate more fees and are seeking to provide services to new potential clients, how do they approach them or get approached? If for example the bank thinks that company XYZ would be a good fit for a sponsor, even if the company has no interest in selling or neither the sponsor has an interest in buying (initially), do banks still just approach the company, negotiate and pitch? I am just curious how the process works if the service is not in demand and the bank is seeking to make more money... This maybe a stupid question idk but I would seriously appreciate any insight as this is still vague to me...