In openLCA you can connect product and waste flows of processes in a product system. When you have a process `A` that has an input of a product `b` and a process `B` that produces `b` as an output, you can connect this input of `b` in `A` with the output of `b` in `B`. Note that `b` needs to be a product flow (you select type = `Product flow` when you create that flow). For this example, you can then select `B` as the provider of the input of `b` in `A`.
For waste flows it is the other way around: you can connect the processes `A` and `W` when the process `A` has an output of waste flow `w` and the process `W` has `w` as an input. You can then select `W` as the default provider for the output of `w` in `A`. In openLCA, you cannot connect processes by elementary flows as these flows describe the direct exchanges of the processes with the environment related to the produced and consumed product and waste flows.