Q1: https://leetcode.com/discuss/general-discussion/399344/dependency-graph
Q2: System Design: Design Netflix
Q3: https://leetcode.com/problems/cheapest-flights-within-k-stops/
Q4: https://leetcode.com/problems/median-of-two-sorted-arrays/
Got an offer after 5 days and accepted! I got rejected from Amazon twice before and Facebook like earlier this month. My advice would be to keep working through the problems and honestly understand the basics.