If I understand correctly the weekdays of Advent, Christmas, Lent, and Easter Season; solemnities, feasts, and memorials are not to be celebrated? Or which are celebrated?
To my knowledge you understand incorrectly. The local Ordo shows weekday memorials (optional and obligatory), feasts and/or solemnities during each of the four periods you cite.
There are plenty of solemnities, feasts, and memorials that always fall within fall within the Advent or Christmas seasons. And there are other solemnities, feasts, and memorials that usually/always fall within Lent or the Easter seasons. If they weren’t celebrated on the usual day then they could never be celebrated.
However those solemnities, feasts and memorials might be suppressed or transferred if they fall on a Sunday of one of those seasons or if they fall in Holy Week.
Speaking of, isn’t Immaculate Conception moved to Monday this year, but is not a Holy Day of Obligation in the US?
Yes. I wondered if that was part of what sparked the OP’s question.
It does seem to be a tricky issue at times. Even a long-timer like me has to refer to the calendar at times. This is my understanding: Solemnities take precedence over most Sundays except during Advent and Lent. For example, if the Assumption falls on a Sunday in August, we celebrate the Assumption instead of the regular Sunday in Ordinary Time. December 8 always falls in Advent so the Immaculate Conception has to be celebrated on a weekday instead of Sunday.
When Feasts or Memorials are superseded by a day of higher rank they are omitted. When a Solemnity is superseded it is transferred the next available day.