To better understand how a sentence can be converted from one way to another I think is very important to be informed of some other important grammar aspects like for example the clauses that somehow are linked to this topic so it can be useful and easier to understand when studying or practicing simple or complex sentences.
To convert a simple sentence into a complex sentence we basically need to add extra information to it and once it happens it can be a noun clause , adjective clause or adverb clause so that is the reason why clauses are linked to this topic.
In other words, a complex sentence emphasizes or provide further details that usually a simple sentence does not have but at the end they express the same meaning. For instance, the way I can relate myself to this topic and I think it could be useful for people learning English , is when you are talking to an English native speaker and they said a sentence that you cannot understand very well therefore you ask "Could you repeat that again please?" so at that moment they noticed that for their message to be understood in a better way they will need to use simpler words so it can be easy for the receptor to get the message and basically in that case complex sentence is being converted into a simple sentence. It is not exactly how this conversion works but it helps us to have at least an idea of how it is and how it can be related and useful in daily activities.