Explicitness and implicitness have nothing whatsoever to do with externality and internality.
If it is my will to eat this apple here, being cognizant of this as an object of thought, it is an explicit intention, whether or not I announce the fact or ever make it known.
Something implicit is something that is implied - I’m not sure that an “implicit intention” is an intention at all, but more a consequence of an explicit intention - maybe if I intend to get from point A to point B, not knowing that I need to take the bus, taking the bus is an implicit intention - though there is no guarantee I will intend to take it when I find out I need to!
Well, an online dictionary says that intention is something that is determined mentally. And something determined mentally is obviously something determined internally. Because, matters of the mind are internal matters. And, something explicit is described in the same dictionary as something clearly developed or formulated. Something with a clear developed form is something that in both cases can be observed with at least one of the five senses. And, the five sense deal with exterior or objective matters, not interior or subjective matters.
You provide a useful way of thinking about explicitness and implicitness with your point A to B analogy. But, you err by rejecting the interior nature of intention. Here is how I am thinking about this. Our fate is not in our own hands. Our fate is in Gods hands. As a consequence, none of us can truly manifest and explicit intention. One could say "I intend to receive the sacrament of baptism by water at the end of my catechism formation". However, God could cause an earthquake that results in the death of such person. And, that would determine such intention as only an implicit intention, because such person did not know how to get from A to B. They didn't even know it is entirely in Gods hands. So much for that soul's explicit A to B intention. So, an explicit A to B does not exist for the catholic. Intention is an implied matter. And, something implied as with something intentional, is something that is subject. And that something subject is something subject to us.
God is the author of our lives. We can never exhibit an explicit intention. That is why catholics often say "God willing" when we speak of our intentions.