The word saṅkhāra is an important doctrinal term in Buddhism. It has a wide variety of meanings, which is usually an indication of a long history. Here is a quick indicative survey of uses before the Buddha, in the primary pre-Buddhist texts, presented in rough chronological order. I’ll do a bit of a summary at the end.
survey
Rig Veda
RV 1.38.12
susaṃskṛtā abhīśavaḥ
Let your reins be well-fashioned
RV 5.76.2
na saṃskṛtam pra mimīto
The well-performed/prepared (offering)
Not really clear to me!
RV 6.28.4
saṃskṛtatra
“readying-place”, slaughterhouse, bench for slaughtering animals for the sacrifice
RV 8.33.9
raṇāya saṃskṛtaḥ
readied for battle
RV 8.77.11
ubhā te bāhū raṇyā susaṃskṛta
both arms readied for battle
Atharvaveda
AV 4.21.4a
na saṃskṛtatram upa yanti
They do not lead them to the slaughterhouse
AV 11.1.35c
sukṛtāṃ loke sīda tatra nau saṃskṛtam
rest in the well-made (heavenly) world that is prepared for you
Śatapatha Brāhmaṇa
SB 1.1.4.10
saṃskuru sādhusaṃskṛtaṃ saṃskurvityevaitadāha
Do thou prepare this oblation for the gods! do thou prepare it thoroughly!’ thereby saying, 'Get this oblation ready for the gods! get it quite ready!
SB 3.2.1.22
tasmādu strī pumāṃsaṃ saṃskṛte tiṣṭhantamabhyaiti
Therefore a woman goes to a man in a well-built house
SB 3.2.3.23 (and 3.4.1.26)
pūrvārdhamevaitadyajñasyābhisaṃskaroti
the fore-part of the sacrifice he perfects (performs?) by this
SB 3.3.4.16
yajamānasya gṛhāngaca tannau saṃskṛtamiti nātra
Go to the sacrificer’s dwelling,–that is the place prepared for us.
SB 3.4.4.14
vajramevaitatsaṃskaroty
he thereby constructs the thunderbolt
SB 3.4.4.17
saṃvatsaramevaitatsaṃskaroti
it is made of the form of the year
(i.e. the ritual performed corresponds with the year with its seasons.)
SB 3.5.4.24
śiraḥ saṃskṛtam
The head (of the sacrifice) is complete.
SB 4.2.1.27
sa prathamā saṃskṛtirviśvavārā
This is the first consecration, assuring all boons
SB 4.3.4.5
etasyātmānaṃ saṃskurvantyetaṃ yajñamṛṅmayaṃ yajurmayaṃ sāmamayamāhutimayaṃ so 'syāmuṣmiṃloka ātmā bhavati
they prepare him another self,–to wit, this sacrifice, consisting of Ṛc and Yajus and Sāman and oblations,–that becomes his self in yonder world
The action of the ritual creates a new or transformed self in the next life.
SB 6.2.1.5 (also 9)
ime vā agnirimānevātmānamabhisaṃskaravai
They are Agni: I will fit them unto mine own self
Seems to be about transforming or perfecting one’s self by analogy with the ritual?
SB 6.2.1.27 (and 33)
saṃskuryādetaṃ
make him complete
SB 6.3.1.15
saṃskariṣyanto bhavanti
they restore/prepare
SB 6.4.2.6
triṣṭubātmānamevāsyaitābhyāṃ saṃskaroti
the Triṣṭubh is the body (self): it is his (Agni’s) body he makes up*
SB 6.5.3.7
rūpaṃ tadasya tena saṃskarotyatha
he thereby creates that form of his (Prajapati’s)
SB 6.7.2.6
garutmānvīryamevainametadabhisaṃskaroti
he therefore creates him so as to be endowed with vigor
I.e. performs the ritual in the proper way, thus endowing the ritual with the qualities of the Garuda who is energy (vīrya).
SB 6.8.1.4
mānuṣaṃ rūpaṃ tadasyatena saṃskaroti
he creates the human form (or divine form)
SB 6.8.2.1
sadātmānamabhisaṃskariṣyāmahe maryāḥ kuṇapā
If we create this, such as it is, part of our own self, we shall become mortal carcasses, not freed from sin
SB 7.1.2.11 (a common idiom)
sarvaṃ kṛtsnaṃ saṃskaty
he restores him so as to be whole and entire
SB 7.1.2.21
sarvāṃ vācaṃ sarvam prāṇaṃ sarvamātmānaṃ saṃskurute
makes up for himself the whole Vāc (speech), the whole vital air, the whole body (of Prajāpati)
SB 7.2.1.1
imameva taṃ lokaṃ saṃskṛtya samārohaṃste
it is this world they (the gods) ascended after completing it (the sacrificial fire)
SB 7.5.2.32 (also 33, 34, 35, 36)
tanūstena cinvāna ātmānaṃ saṃskuruṣvetyetanmayuṃ
the form is the self: thus, 'Building up therewith, perfect thyself
Obscure to me, but related to the idea of the substitute sacrifice. The sacrificer begs Agni to accept the kimpuruṣa (“what man?”). The translator says it is unclear, perhaps a fashioned facsimile or else a monkey.
Chāndogya Upaniṣad
CU 4.16.2
manasā saṃskaroti brahmā
The brahma (priest) acts by mind
summary
The overriding sense is to “make” or “create” or “fashion” something, especially in a good way, to “perfect” (which is in fact a close verbal parallel, “full-make”).
The oldest uses in the Rig Veda tend to emphasize the making or construction of some physical item, a use echoed in, say, Dhp 154, or the sense of “readied, prepared”.
Over time the sense of “prepared” (for the sacrifice) came to dominate. But this is really just a reflection of the kind of literature, which concerns the ritual, while presumably in secular usage it continued to have the sense we see in the Rig Veda. This usage is extremely common in the Śatapatha Brāhmaṇa, these are just a selection of quotes.
This then leads to a more pregnant philosophical sense, where the action of the sacrifice builds or creates or transforms the self, by analogy or identity, so that the created self goes to heaven. This is evident in such Pali contexts as MN 120, although obviously the sacrifice has been replaced by ethical action.
The sense of “all conditioned things" is not found at all, and should be understood as a specifically Buddhist dialectic: “The world of creation embodied in your ritual (which you have worked so hard to get just right under the assumption that it is the eternal emanation of divinity) is really just the product of many different choices and actions, all of which are impermanent and corrupted, and which create a phenomenal world that’s just as bad”.
The positive connotation of saṅkhāra in the sense of “well-made, properly prepared” is an assumed ironic substrata of this:
Alas, perfections are impermanent!
I.e., “What you think is so well made is falling apart”.
In Buddhism the idea of a creative action being the ritual was completely replaced by ethical action, i.e. making good moral choices. This sense, as so often, rapidly dominated, so that later generations of Buddhists forgot the context in which the Buddha was speaking.