It's not completely different. They appear to be identical in large portion. Check the diagrams at: https://patents.google.com/patent/US6630507B1/en

that's not the way screens work. definitely not the way immunoassay screens work, which is the most common screen that isn't just making shit up.

confirmatory testing, like a GC/MS or LC/MS/MS can tell them apart because they're different molecules, but the extraction processes for delta 9 THC can/will pick up all the similar molecules (delta 8, COOH, CBD, CBN, whatever).

that's necessary and good because they're not all psychoactive (the COOH in particular), so they've got to be separated out for forensic purposes.