When the ark was completed, the glory cloud of the Lord (the Shekinah
Glory) covered the tent of meeting, and the glory of the Lord filled the tabernacle (Ex. 40:34-35; Num. 9:18, 22). The verb for "to cover" or "to overshadow" and the metaphor of a cloud are used in the Bible to represent the presence and glory of God. The Catechism
In the theophanies of the Old Testament, the cloud, now obscure, now luminous, reveals the living and saving God, while veiling the transcendence of his glory — with Moses on Mount Sinai, at the tent of meeting, and during the wandering in the desert, and with Solomon at the dedication of the temple. In the Holy Spirit, Christ fulfills these figures [types]. The Spirit comes upon the Virgin Mary and "overshadows" her, so that she might conceive and give birth to Jesus. On the mountain of Transfiguration, the Spirit in the "cloud came and overshadowed" Jesus, Moses and Elijah, Peter, James and John, and "a voice came out of the cloud, saying, 'This is my Son, my Chosen; listen to him!"' Finally, the cloud took Jesus out of the sight of the disciples on the day of his Ascension and will reveal him as Son of man in glory on the day of his final coming. The glory of the Lord "overshadowed" the ark and filled the tabernacle (CCC 697).
It's easy to miss the parallel between the Holy Spirit overshadowing the ark and the Holy Spirit overshadowing Mary, between the Ark of the Old Covenant as the dwelling place of God and Mary as the new dwelling place of God.
God was very specific about every exact detail of the ark (Ex. 25-30). It was a place where God himself would dwell (Ex. 25:8). God wanted his words — inscribed on stone — housed in a perfect container covered with pure gold within and without. How much more would he want his Word — Jesus — to have a perfect dwelling place! If the only begotten Son were to take up residence in the womb of a human girl, would he not make her flawless?
The Virgin Mary is the living shrine of the Word of God, the Ark of the New and Eternal Covenant. In fact, St. Luke's account of the annunciation of the angel to Mary nicely incorporates the images of the tent of meeting with God in Sinai and of the temple of Zion. Just as the cloud covered the people of God marching in the desert (cf. Num. 10:34; Deut. 33:12; Ps. 91:4) and just as the same cloud, as a sign of the divine mystery present in the midst of Israel, hovered over the Ark of the Covenant (cf. Ex. 40:35), so now the shadow of the Most High envelopes and penetrates the tabernacle of the New Covenant that is the womb of Mary (cf. Luke 1:35) (Pontifical Council for the Pastoral Care of Migrants and Itinerant People, The Shrine: Memory, Presence and Prophecy of the Living God