What is the Catholic understanding of the idea that Jesus descended into Hell? The Christian understanding that everyone was born with origin sin, and then when Jesus/God was killed at the cross, and then he was resurrected from death after 3 days.

From what I understand, there is the idea of Jesus descending into Hell to free the spirits/souls of people.

Who were these people?  Was it everyone before his time that died?
Did it include the righteous and wicked people or prophets also?

Is this a creed based on belief only or is there scripture to back this up somewhere?
First, be careful with your statement &quot;The Christian understanding that everyone was born with origin[al] sin&quot;. Yes, this is a common Christian belief, and it is indeed a Catholic one, but it is not universal. This is irrelevant to the rest of your question, but worth noting.
And yes, there does indeed exist the idea that Jesus descended into &quot;Hell&quot; between his death and resurrection, i.e. between the first Good Friday and the first Easter morning. We see this in the Apostles' Creed, for instance: &quot;He descended into Hell&quot;. This is an event widely known as the Harrowing of Hell.
Now, the first thing to note is that this is a very different thing to what we imagine Hell to be like today. It does not mean a place of everlasting torment. It's sometimes known as the &quot;Limbo of the Fathers&quot;. In Hebrew it is Sheol, the abode of the dead. As the Catechism says, &quot;Such is the case for all the dead, whether evil or righteous, while they await the Redeemer.&quot;(CCC 633)
It was precisely the just -- those made righteous by faith in God and his promised Messiah -- who were saved by Jesus' descent into Hell: &quot;Jesus did not descend into hell to deliver the damned, nor to destroy the hell of damnation, but to free the just who had gone before him.&quot;(CCC 633)
This emphasises the totality of Jesus' redemptive work. It extends not only to those on earth, over whom God is known to have power, but also to those who have died.

The descent into hell brings the Gospel message of salvation to complete fulfilment. This is the last phase of Jesus' messianic mission, a phase which is condensed in time but vast in its real significance: the spread of Christ's redemptive work to all men of all times and all places, for all who are saved have been made sharers in the redemption.(CCC 633)

See also this article on EWTN.

As to its biblical foundation, there are various texts that we might point to.
The Catechism points to the following texts, among others:

When it says, ‘He ascended’, what does it mean but that he had also descended into the lower parts of the earth? (Eph 4.10)
you killed the Author of life, whom God raised from the dead (Acts 3.15)
If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ from the dead will give life to your mortal bodies also through his Spirit that dwells in you. (Rom 8.11)
He was put to death in the flesh, but made alive in the spirit, in which also he went and made a proclamation to the spirits in prison (1 Pet 3.19)
For this is the reason the gospel was proclaimed even to the dead, so that, though they had been judged in the flesh as everyone is judged, they might live in the spirit as God does. (1 Pet 4.10)
Very truly, I tell you, the hour is coming, and is now here, when the dead will hear the voice of the Son of God, and those who hear will live (Jn 5.25)