How does weed killer get INTO the plant (leaves? roots? stem? mechanism?)

After cutting a 500-yard long path through thick poison oak, I'm now
applying huge amounts of glyphosate-based Roundup weed killer to kill
remaining emergent plants left behind after the battle.

These constant skirmishes are depleting my Roundup, gallon by expensive
gallon (bought in concentrated bulk from Costco).

I'm spraying the stems, the ground, and the leaves ... but ... I wonder ...
what is the mechanism that allows UPTAKE of the glyphosate?

I looked up how weed killers kill, and can easily ascertain that
glyphosates mimick the natural EPSP Synthase needed as a catlyst for the
plant to create proteins ... but nothing I've found so far tells me how the
glyphosate is ABSORBED into the plant and WHERE it is best applied
(leaves?, stem?, roots?).

Mostly, I'm left with huge tangles of poison-oak stems ... so I ask ...
WHERE does weed killer get INTO the plant and how?