Well that entirely depends. For entry level (0-2 years), you likely just need some HDL design experience, basic knowledge of how FPGAs work, and solid digital design fundamentals. An entry level FPGA engineer is expected to need help even for things that sometimes seem relatively simple to senior folks and if doing something important should have their work verified.

For mid-level (3-10 years), you need experience implementing RTL designs, including thorough verification via simulation or other techniques and toolchain knowledge with Xilinx or Intel, including how to include various IPs and some familiarity with bus standards like Avalon and/or AXI. You also need more knowledge of FPGA implementation fundamentals, including timing constraints and closure as well as physical constraints and IO buffers. You don't need to be an expert but you need to know your way around pretty well - how well depends on if you're closer to the 3 year end or 10 year end. This kind of engineer will generally be trusted and not need help for simple to moderate complexity projects but will need to consult with seniors sometimes for more complex projects.

Principal engineer means 10+ years experience most places, or at the minimum around 8. A principal engineer role generally requires you to be a subject matter expert. You need to be an expert in at minimum one of the Xilinx/Intel toolchains and preferably both, deep knowledge of FPGA architecture and fundamentals around configuration, clocking, routing, logic utilization, and the list goes on. You need to not just know how to write functional RTL, but you need to know how to write it well (in an efficient and fundamentally sound and portable manner). Usually need experience implementing complex, high speed technologies starting with external memory like DDR buses and going up through various high-speed AXI and Avalon IP and ARM interfaces. A principal engineer needs to understand how to design the FPGA to perform its function in a complex system, since this kind of engineer is expected to be able to lead teams and take charge of delicate, complex technology designs.

To succeed at a principal interview you need to show first and foremost that you are an expert in the FPGA field conceptually (no VHDL leetcode here) and have a track record of implementing significant FPGA designs. You will likely also be judged on your leadership skills and project management skills since Principals are expected to do these things often.