German "Streusel" is just the crumbly dryish component covering a cake or pastry made from a little flour, butter and lots of sugar. Streuselkuchen is a cake made with yeast dough covered in Streusel. Optionally, one can enhance this with fruit or vegetables, e.g. rhubarb. But cheesecake, cheese pastry (with a flaky pastry base) or fruity pastry can also be covered with some Streuseln.
Strudel is a bigger (generous 1 portion or more) piece of pastry (usually puff pastry, but plain pastry dough is also not unusual) with fruit filling, commonly apples and raisins. Often served with vanilla sauce.
Of those, I've usually seen strudel to have the original German meaning while streusel in english is usually used to mean Streuselkuchen with fruit.
I think you’re confusing two things. Strudel seems to be strudel as well in English, while Streusel is a kind of crumble, it’s not a dish by itself.