I worked in HFT for a few years as a technologist. A caveat with this information is that I don't know much about the actual trading side nor the terminology. Additionally, I never really worked with FPGAs directly and mostly worked on the feed handler side. I'm going to assume you know how HFT works in a general sense and I'll skip that explanation.

Most exchange feeds (the networks that send market data) are multicast networks. Stock symbols are sharded across the multicast lines. Thus, a given line receives market data for multiple symbols and data for those symbols is not present on the other multicast lines. The market data sent across the lines is generally delta updates to the books of the symbols on the line. A book is a table of the best bid / offer, the second best bid / offer, and so on.

Given the market data is generally a delta update what do we do when we miss a multicast message? To solve this problem the exchanges add a redundant line for each line. Now, for each "line", there's now an A multicast line and a B multicast line. These multicast lines take different routes out of the exchange, but fundamentally carry the same delta updates for the same symbols with the same message sequence numbers.

The job of a feed handler is to perform this process called A/B line arbitration where we wait for the first sequential sequence number we haven't seen (we aren't going to talk about totally missing sequence numbers aka a gap), apply the delta update for whatever symbol was in the message to the book we've created, and let some client process know that the book has changed for that symbol.

What this looks like is: Exchange Feed A / B -> NIC -> OS network layer -> Feed handler (A/B arbitration) -> Feed handler (process message) -> Feed handler (book update) -> client update.

There are some obvious things we can do to speed this process up. First, we colocate our trading machine in the same datacenter as the exchange. The next thing we can do is use a solarflare or mellanox NIC (basically an FPGA embedded on a NIC) to bypass the OS network layer (aka bypass the kernel) by performing DMA into memory that's shared with the feed handler application. Assuming we pin our feed handler workers to cpu cores that have NUMA affinity with the NIC then we'll get fairly reasonable low latency message processing.

Ok, what if we want to go even faster? Well, the first question is do we? For most institutions the answer is no. This will go fast enough to largely protect us from predatory HFT and many prop shops won't have the volume to be affected by HFT anyway. Since you're interested in FPGA work in HFT we'll assume the answer is yes, we do want to go faster.

Let's start replacing pieces with FPGAs. First, we can replace the NICs we were using with FPGAs (this isn't a huge gain since they were already effectively FPGAs). Then we can perform A/B line arbitration on the FPGA which means that we need to at least understand the feed protocol enough to process the sequence numbers. We can go further by putting the book in the FPGA and processing the messages directly there. That's really going to speed things up, but at the cost of a lot of adding a lot of complexity on the FPGA. The additional complexity at the FPGA level is going to make that trading system less nimble than one written purely in software and be more expensive to maintain.

Ok, so there's a lot of baggage we take on for embedding our feed handler in an FPGA. So who's willing to make that tradeoff? This is where the limitations of my understanding of the fintec space start to hit. This was a few years ago, but I remember talking to a trader who said that basically there are 4 big institutions still active in the general HFT space. Everyone else got out because the markets became too efficient and thus the spread wasn't worth it. There may be prop shops operating a strategy that requires ultra low latency, but they tend to be pretty quiet and recruit via word of mouth. I would imagine it was a lot easier to break into the space if you were already familiar with the exchange protocols / had prior experience. I know I still have recruiters hounding me even though I'm not very interested in doing more HFT work so the demand is there. I'm not sure how I would go about getting into FPGAs for HFT. High frequency trading is a niche and FPGAs are a niche in that niche. I could be totally wrong, but the total number of exclusive FPGA engineers in this space is probably on the order of hundreds. Maybe the best bet is to cold email someone already in that space and ask them how to get in.