good answer...I am not sure if your questions are answered. Here are my 2 cents.
Yes, Kodak sells C-41 developer (comes in 3 part called Part A, B and C) that is ready to use when mixed. If a developer is not called a replenisher it is one that after mixing part A, B, C and water it will be ready to use. If it is called a replenisher then you will need to mix the part A, B, C and water still as well as mixing some starter.
Why are the two different developers? The replenisher version is for labs to replenishing their running processor that is constantly consuming the key ingredients in the developer. A developer replenisher has the right ingredients to restore the developer back to what a C-41 developer needs to contain.
So a developer replenisher is not the same as a developer. Most mini labs only buy developer replenisher rather than developer. They constantly replenish their working processor as long as the processor is processing films. They will need fresh developer only when they dump the developer and start with a full tank of fresh developer.
They can produce a fresh tank of developer by adding starter to the replenisher they have. So can we. In fact the replenisher is more available than developer from wholesale suppliers.
There are different versions of developer replenishers. There is at least the standard developer replenisher and developer replenisher LORR. The difference between them is that it takes much less quantity (about half) of developer replenisher LORR to replenish. So this is the more preferred version of the replenisher. This is also called C-41B developer replenisher I believe. This is what I use. I bought mine from a mini lab supplier.
Bleach and Fix all come in replenisher package I believe. They are used for replenishing working processors too. But you don't need a starter to make working bleach or working fix. As PE said they can be used without a starter.