There is some weighing of probabilities of other possible explanations (which there always has to be), but there is an important presumptive side to the design inference. Namely, our repeated and uniform experience that, for example, complex, integrated, functional systems come *only* from a process of planning, coordination and design. We see such systems designed regularly; we never see them come about by chance and necessity.
The *only* reason anyone is even arguing about whether the physical systems we see in life are designed (Darwinists regularly admit they look designed and they have to keep reminding themselves that they aren’t designed) is because either (i) folks have a philosophical objection to them being designed, or (ii) they imagine that some unknown, unspecified, as-yet-undiscovered, natural process in the distant past is an exception to our repeated and uniform experience and can somehow create the illusion something was designed even though it wasn’t actually designed.
If we want to talk about weighing probabilities, it isn’t even close. Unless, of course, we have that nasty little philosophical hangup