The Council decided Chat Control was on the agenda, the Commission wrote the law, and the MEPs voted to reject said law in March. Then the president of the parliament (not the MEPs at large) asked the Council to ignore the March vote and proceed with the agenda under urgency as if it had passed.
Now a minority of the MEPs (331 out of 720) - but a majority of who were present at the time and chose not to abstain - have voted to deal with the matter under urgency, but haven't voted on the substance of it. This makes the actual vote happen on the last sitting day, when apparently they are hoping a lot of MEPs will be away.
> The Council decided Chat Control was on the agenda
The Council is different from the European Council (yes, the treaty drafters were not much creative in naming institutions), the latter is composed of the heads of states and sets the agenda like you said, while the former is composed on ministers in the policy area under discussion, and it's a "co-legislator" together for the Parliament (on most areas, including Chat Control 1.0 & 2.0, both must agree to pass an act).
The issue here (it's part of the "democratic deficits") is that, in its second reading, the EP needs an absolute majority to amend/reject the Council first reading, and a simple majority to approve it and pass it into law.