In short - they design IP blocks for either existing IPs or new IPs. Detailed: A typical role will be based on the experience. If the engineer is fresher, then he would be initially tasked to run some design related tools and analysis the design or assist the verification team to fix the bugs. After 6months, the engineer will be asked to work on small RTL designs under a senior design engineer. The work can be small feature updates to a large revamp of the IP architecture which will be complete new design. An experienced candidate usually works independently and own sub-blocks or full IP as is. Microarchitecture for sub-blocks are also done by the designer itself.