How does 'consist' compound with 'in' mean “to have as an essential feature”?

1.1. [consist in] = Have as an essential feature

How did the Latin com- + sistere compound to mean 1.1 above? It may help to read ODO's own conjecture of the etymology (see the link above).

[Etymonline for consist (v.) :] 1520s, from Middle French consister (14c.) or directly from Latin consistere "to stand firm, take a standing position, stop, halt,"
from com- "together" (see com-) + sistere "to place," causative of stare "to stand, be standing" (see stay (v.)). Related: Consisted; consisting.

Solution 1:

The Latin word consisto already had this (additional) sense, according to Lewis & Short:

2. (According to I. B. 2.) To be or remain firm, unshaken, immovable, steadfast, to be at rest, to stand one's ground, to continue, endure, subsist, be, exist ...

...

4. In general, to be, exist ...

4 (b). With in, ex, or the simple ablative (in Quintilian also with circa and inter; vide infra), to consist in or of, to depend upon:

“major pars victūs eorum in lacte, caseo, carne consistit,” Caes. B. G. 6, 22: “omnis per se natura duabus Constitit in rebus,” Lucr. 1, 420: “e quibus haec rerum summa consistat,” id. 1, 236; so with ex, id. 1, 839; 1, 873 al.; with abl.: “deveniunt in talis disposturas, Qualibus haec rerum consistit summa,” id. 1, 1028; 5, 61; 5, 66: “vita omnis in venationibus atque in studiis rei militaris consistit,” Caes. B. G. 6, 21: “in eo salus et vita optimi cujusque consistit,” Cic. Phil. 3, 8, 19: “in hoc summa judicii causaque tota consistit,” id. Quint. 9, 32; cf. id. de Or. 1, 40, 182: “causam belli in personā tuā,” id. Phil. 2, 22, 53: “in quibus vita beata,” id. Tusc. 5, 14, 40: “in unā honestate omne bonum,” id. ib. 5, 14, 42: “in nomine controversia,” Quint. 7, 3, 7; 8, 3, 57: “in actu rhetoricen,” id. 2, 18, 2; 6, 3, 42: “spes omnis consistebat Datami in se locique naturā,” Nep. Dat. 8, 3.—With abl., Quint. 12, 10, 59: “omnis quaestio circa res personasque consistere videtur,” id. 3, 5, 7; 6, 3, 19: “quaestio inter utile atque honestum consistet,” id. 3, 8, 24. —

So the word already had this meaning in classical Latin, which makes its post-classical history irrelevant. Con-sisto literally means "stand together in x" in Latin.

As to how it acquired this meaning in Latin, I do not know.

Adding the prefix con- "together" to the verb sisto, "to (cause to) stand", resulted in the meaning "to go and stand (somewhere), to stop, to stand still".

Lewis & Short (see above) suggest that it more or less went from "stand together, stand firm" to "be/exist" to "be in essence" to "be represented in essence in [a thing or quality]".

But con- also often indicates that the action represented by the simple (= without affixes) verb comes to a natural end, as can perhaps be seen in conclude, convert, confer. The end 'comes together' with the beginning of the action, as it were. Sisto/sto "stand" is often close in meaning to "be" and "exist", as can be observed in existo as well, which means "to stand out, to exist".

It is possible that consisto went from "stand together" to "stand complete" to "be complete" to "be in essence" to "be represented in essence in [a thing or quality]". But perhaps we would be wiser to follow Lewis & Short.

I don't think the exact semanto-etymological path is known.

Note that a somewhat similar path is also present in Germanic, as in Dutch:

staan = to stand

bestaan = to (continue to) exist

bestaan uit = to consist of

bestaan in = to consist in

Perhaps this originates in a loan translation from Latin. That is certainly possible for bestaan uit and bestaan in, but hardly for bestaan "exist", because that use already existed in Old Dutch and Old High German.