usually one/two are in charge of anonymising the treatment. The fewer, the better to keep things clearer. They will track how they've anonymised it, store that information elsewhere, and then destroy evidence used during the process (like any quick notes made during).

Other researchers then perform the actual experiment/trial and track the anonymised notation (symbols/letters/numbers) with the experimental info.

You have to remember, the anonymisation is only being performed at the behest of THE RESEARCHERS. THEY (or bodies related to the researchers) want to know that A works and B does not, because every dead end cut off early is money saved. They're not going to circumvent their own process for... no apparent gain (unless they are a biotech startup with unrealistic goals in Silicon Valley happy to fake real patient data, I guess... thats another story).

That said... sometimes the immediate observable results kind of give away which treatment might be which, so its not always avoidable.