He placed the shirt upon him, and tied the sash/avneit around him.
Meshech Chochmah: We know the fabric content of the kohen gadol’s avnet, because it is described explicitly in the Torah as containing both wool and linen thread. We have no such description for the avnet of the common kohen. According to one opinion, the avnet of the commoner did not contain the usually forbidden shatnez mixture. Tosafos explain that it is reasonable to limit the surprising allowance of shatnez to the kohen gadol, whose other garments (ephod, choshen) also contained shatnez, and exclude the allowance from ordinary kohanim, whose other garments were of plain linen, without any admixture of wool.
The deeper meaning behind this distinction stems from the function of the bigdei kehunah, which is to serve as kapparah for certain sins. The avnet is linked to thoughts of sin. Now, the garments of the common kohen atoned for murder and for illicit relations, both of which are active transgressions. The garments of the kohen gadol, however, included the ephod, which atoned for avodah zarah. This last transgression differs markedly from the others, in that it is primarily a transgression of mind and attitude; it can be violated without any active manifestation. Sinful thoughts of avodah zarah are thus much more significant than those of other aveiros, in that they are part and parcel of the primary sin. This means that the sins of mind addressed by the avnet of the kohen gadol are on a different plane than the thoughts of sin of the common kohen addressed through his own avnet. The latter show a deficiency that needs addressing, but not a deficiency as deep as that of thoughts of avodah zarah. The Torah underscores the difference by making the two avnets of different materials. Since the kohen gadol wore an ephod that atoned for the most serious inner transgressions of mind and thought, his avnet matched the ephod he wore at the same time.
We can take the discussion further. Ultimately, all sins are branches reaching out from three roots, i.e.the three cardinal sins. All sins of lust and desire are sourced within the sin of gilui arayos. Sins that involve jealousy of and harm to others are related to murder. That leaves avodah zarah as the source of all aveiros between Man and G-d.
On Yom Kippur, we are told, Soton holds no sway over us. Yet the gemara speaks of many people violating young women on this holiest of days itself! We must realize that we are freed on Yom Kippur from only one class of aveirah. By desisting from eating, drinking and other activities, we downplay the importance of the physical, allowing us to become more angelic, for our spirits to return to their Divine source. The kohen gadol dramatizes this by taking his avodah into the kodesh kodashim itself, symbolically restoring our neshamos to the place from which they came.
In such an environment, there is no room for transgressions of the mind and spirit. No need on such a day for atonement for thoughts of avodah zarah. The kohen gadol wears no ephod in that inner avodah; his avnet is of pure linen. Man’s spirit is elevated by the day itself; what remains are the coarser elements of the nature that we share with lower animals. The urge to violate young women, coming from this more primitive part of ourselves, is not naturally quashed. Coarser individuals succumb even on Yom Kippur!
Yom Kippur, we learn, is like a mikvah, purifying those who immerse themselves within it. We have seen how the avodah of the kohen gadol in the Holy of Holies expresses this. We use the immersion procedure in yet another way. The Jewish people as a mystical entity – Knesset Yisrael – remains always connected to HKBH. In the days before Yom Kippur, we multiply acts of tzedakah and chesed, immersing ourselves, so to speak, more deeply into a union with that entity. By doing this, we become like branches grafted on to a tree, becoming one with the trunk itself, and through it, to Hashem as well.
 Based on Meshech Chochmah, Vayikra 8:7
 Yoma 5B
 Yoma 6A s.v amar
 Zevachim 88B
 Yoma 20A
 Yoma 19B
 Yoma 85B