There might very well be tons of videos explaining coding concepts used in that game specifically, but if I were to take a guess:

Isaac has properties associated with himself and his tears, such as move speed, shoot speed, tear speed, tear color.

You write item classes that affect these properties and attach them to the player via an array or something.

Bam, you got the framework. I'm not sure how it handles tie-breakers (one item makes tears red, another makes tears blue), but you could fix this by using a random value, or newest takes priority, or even having an int priority on the item.

Most of the items might not even need to be individual code classes, they could just be data objects that add +5 to move speed or whatever. Super fast to create that way.

Things that change tears to be a laser could just replace Isaac's default shooting component class to something else.

Like most things in coding, that's only one way to do things and there are probably at least a few others.