A spreadsheet on my computer would pull data from Bloomberg using their Excel API and would price the securities. Those prices would be pulled off the spreadsheet by some Reuters application that would then send those numbers through a 56K modem to Reuter's system. Somebody I never met in London would screen scrape the prices from Reuters into Excel. From that Excel spreadsheet the prices would be posted on the Swiss exchange. I was responsible for any resulting trades and they would go into my book.
These were two sided prices (bid-ask) so if the process broke and the market moved I would get picked off. Thankfully they were mostly all delta 1 instruments with 50 bps of spread so that didn't happen too often.
or
This is not an apples to apples comparison to the spider fight world of trading, but there would still be a lot of ingenuity there. And a simple way to limit risk is to ruthlessly review the results, both automatically and manually. Keep going while it works as expected and stop/adjust if there is a major glitch.
A crappy solution was still the best they had at the time. You're going to hate to read what they say about today's solutions in 2028.
If it works...
I was usually called in to debug these things. My approach was always super minimalist... How do I make the 1 change that will get this all back on its feet again? .... without looking at any of the VBA or the weird DLLs lurking in the shadows.
Having said all that - Google Sheets is actually a different beast. I still wouldn't trade through it. Also, I've been eyeing Alpaca for a while - I like what they're doing. Best of luck to them....