So I could enter NI from Scotland without having to be tested/vaccinated and wouldn't have to isolate?
Correct.
Unless you're only making a day trip, there's a recommendation to take a free lateral flow test before your journey, and on days 2 and 8 of your stay if your stay lasts that long. There's no legal requirement to do so, however, and you won't be asked for proof.
Vaccination is irrelevant.
And If I were to travel to ROI to NI? Do they carry out checks at the border? Is there a simplified list of restrictions for ROI?(I can't seem to find any anywhere).
They don't carry out checks - the border is fully open as it always was - but if caught (next to impossible) you're subject to fines, a prison sentence, and possibly immediate imprisonment in hotel quarantine at your cost and forced testing (if you don't have one of the three proofs).
You must have one of: proof of full vaccination; proof of a positive test between 11-180 days ago; a negative PCR test either taken no more than 72 hours before entering Northern Ireland
or no more than 72 hours before crossing the border. You must carry this at all times.
If you have one of the first two you're free to go about your business. If you have the latter, you need to quarantine for 14 days, which can be shortened by taking a free test (of the painful, six inches down the back of your nose into your throat, performed by a medical professional, variety) on day 5.
You also must complete the passenger locator form before entry, and carry the receipt at all times.