Hi, I would like to understand which foods are chametz for ashkenazim during Pesach.

Any product that does not specify on the label that it is Kosher for Ashkenazim (has no Kitniyot or rich Matza) you should not eat. Nowadays every food is made from a large and complex set of materials and a simple buyer cannot know which is Kosher and which is not. You can find a list of Kosher products on the OU site. Rabbi Yitzchak Grinblat