Theory of Constraints views all processes as a chain of events that executes sequentially. Remember how a chain is only as strong as its weakest link? Well a 5 step process is only as productive as its slowest step.
It does not matter if steps A and B can output 15,000 products a day because step C can only output 10,000. In addition even if steps D and E can handle more than 10,000 a day that’s still all they get handed off from C. So step C is a constraint on the chain/system.
Theory of Constraints views the extra 5,000 products a day produced by steps A&B as waste, the cost of both producing and storing the extra parts is waste. Any extra machinery/people in A&B or D&E are waste. A common problem with Lean Six Sigma is project selection, often improving the efficiency of step A or D because they find “low hanging fruit”. These improvements are pointless because the system can still only function as well as step C so the cost of those projects is also waste.
Theory of Constraints helps you find the constraints, Lean (and ToC) helps you improve them, Six Sigma helps you eliminate any errors. Additionally ToC introduces buffers to protect your constraint, so even if a machine in step B goes down you still have a little buffer of parts for step C to work on until it’s back up (remember the whole process can only work as fast as step C so if you don’t keep it busy then steps D&E will run out of things to do and stop). Buffers and ToC have a ton of other advantages so I highly recommend you look it up further.