The depositum fidei is the fullness of God's Word contained in both Sacred Scripture and Sacred Tradition. Christ has entrusted to the shepherds of God's people the task of interpreting His Word for the Church:
Sacred tradition and Sacred Scripture form one sacred deposit of the word of God, committed to the Church. Holding fast to this deposit the entire holy people united with their shepherds remain always steadfast in the teaching of the Apostles, in the common life, in the breaking of the bread and in prayers (see Acts 2, 42, Greek text), so that holding to, practicing and professing the heritage of the faith, it becomes on the part of the bishops and faithful a single common effort.
But the task of authentically interpreting the word of God, whether written or handed on, has been entrusted exclusively to the living teaching office of the Church, whose authority is exercised in the name of Jesus Christ.
(Dei Verbum, Dogmatic Constitution on Divine Revelation, article 10; emphasis added)
This teaching office, exercised in the name of Christ, is known as the Magisterium, and is part of the gift of the Word to Christ's Church. It is guided by the Holy Spirit to preserve it from error:
This teaching office is not above the word of God, but serves it, teaching only what has been handed on, listening to it devoutly, guarding it scrupulously and explaining it faithfully in accord with a divine commission and with the help of the Holy Spirit, it draws from this one deposit of faith everything which it presents for belief as divinely revealed.
(Dei Verbum, article 10)
To address your last question more specifically, yes, the Church (through its pastors, that is, the pope, the bishops, and the pastors of parishes) does teach authoritatively about the understanding and foreshadowing of Jesus Christ given us in the Old Testament:
The principal purpose to which the plan of the old covenant was directed was to prepare for the coming of Christ, the redeemer of all and of the messianic kingdom, to announce this coming by prophecy (see Luke 24:44; John 5:39; 1 Peter 1:10), and to indicate its meaning through various types (see 1 Cor. 10:12). Now the books of the Old Testament, in accordance with the state of mankind before the time of salvation established by Christ, reveal to all men the knowledge of God and of man and the ways in which God, just and merciful, deals with men. These books, though they also contain some things which are incomplete and temporary, nevertheless show us true divine pedagogy. These same books, then, give expression to a lively sense of God, contain a store of sublime teachings about God, sound wisdom about human life, and a wonderful treasury of prayers, and in them the mystery of our salvation is present in a hidden way. Christians should receive them with reverence.
God, the inspirer and author of both Testaments, wisely arranged that the New Testament be hidden in the Old and the Old be made manifest in the New. For, though Christ established the new covenant in His blood (see Luke 22:20; 1 Cor. 11:25), still the books of the Old Testament with all their parts, caught up into the proclamation of the Gospel, acquire and show forth their full meaning in the New Testament (see Matt. 5:17; Luke 24:27; Rom. 16:25-26; 2 Cor. 14:16) and in turn shed light on it and explain it.
(Dei Verbum, articles 15–16; emphasis added)
The Church, as the authentic interpreter of all of Sacred Scripture, declares the true meaning of the Old as well the New Testament.