If you use a retail platform, they do not make markets. For example, CMC markets gets the FX rate from liquidity providers like Deutsche Bank, JP Morgan, Barclays, Goldman, UBS, Citibank and HSBC. You should find a similar logic with others.
If you setup a limit order it will simply sit there and get filled on your behalf (aims to fill at the best available price) if the price reaches that level. What you will see on retail market limit order books will be (in my opinion) entirely useless to gauge the buy and sell interest in Forex markets. It will show you what the retail investors using this platform do, but that will be like a sand grain on a beach compared to the size of the FX market, which according to BIS is $6.6 trillion per day.
Spot FX is OTC (over the counter). You request a quote and get a response from a counterparty. Sometimes it is a directly executable (always on) feed of bid ask quotes but the general idea is that you indicate that you want to trade, and the counterparty quotes the price you get. There are some exception like Refinitiv's FX Matching which is essentially a single order book but these still consist of central limit order books (CLOBs) which enable participants to “make an offer” (e.g. place a bid in the market).
My explanation above was misleading. The last paragraph is not retail trading. A constant always on executable quote is almost the opposite of an ECN. It is a direct connection to the market maker(s) of your choice were you click on the quote if you decide to trade (hence accept the quoted price).
Generally, I think you overcomplicate things. FX (especially spot) is largely unregulated (out of scope of MiFID II, MAR, EMIR and MiFIR, not covered under CFTC or the SEC...). Trading is predominantly OTC and dominated by large banks. Retail is according to BIS Table 5 3.3% of total spot trading (66/1987).
As a retail trader you are at the very end of the line. Just like shopping at a super market. Yes, theoretically, the shop decides what to charge for Coca Cola. Realistically, Coca Cola will set the price, and supermarkets simply add a mark-up. Similarly, what your retail broker does exactly is (as long as it is legitimate) essentially almost irrelevant. If they make markets for their clients, it just means they add spreads directly to quotes they receive and do not just pass it on to someone else. Nonetheless, their quotes will be driven by the major banks.
I gave the CMC example, and checked Oanda which also states that they aggregate live prices, in real time, from their liquidity providers. Since I am not familiar with retail platforms, I had to google names for "MM" in retail. Avatrade
writes that it charges 'Spread Over Market' which is the Mark-up AVATRADE adds to the Current Market Spread (which will come from their liquidity provider).
Forex.com generally offer direct links to their liquidity providers too.
Long story short. FX is (predominantly) a quote driven market and dominated by major banks who quote bid/ask prices. Retail brokers will (no matter what they claim to offer) always fall back to these quotes. Just like Coca Cola decides what the cheapest price for the drink will be. All else is mark-up.