smallville is old af. i don't think any of the characters there are considered canon in the current dc tv universe. which is why green arrow from smallville isn't the same green arrow in arrow.
titans on the other hand is supposed to belong to the same universe as the other dc tv shows, they were even supposed to appear in one of the crossovers, but things didn't go through(people assume payment, since hbomax probably pays more than the cw), and the flash from the cw and the flash from the movies have interacted with each other. and since the dctv has merged their universes, that'd mean titans is in the same canon universe as the cw shows and the dceu movies