For someone starting to study Systems Design for interviews, is there a roadmap we should follow so that we understand concepts in increasing order of difficulties?
I don't have a strong opinion here. If I were you, I'd just follow the recommendation provided in a well-regarded system design course or book. Start with ByteByteGo from Taro Perks: https://www.jointaro.com/perks/
Use this as a starting point, but adapt it to topics you feel more or less confident in.