I know that's such a silly subject line :D but I honestly hadn't really thought too much about how they work until now!
Myself (37F) and DH (35M) have just started TTC after getting my copper coil removed. I've got a fairly standard 30-32 day cycle - about 8 months ago I did have a 41 day cycle after starting work in a new office full of women 
#pheromones
Whilst we were waiting during the three months of lockdown for the GP to be able to remove my IUD after lockdown I thought it might be useful to do some BBT tracking using Clue, which sort of indicated that I might be ovulating at around day 18-22 of my cycle.
Cue this month when we first started trying and my expected ovulation week coincided with that week of heatwave and it's sent my temperatures all over the place with no indication of when I might have ovulated (typical!).
So I tested on day 30 because the pregnancy test I bought was describing itself as being 99% accurate - but I'm assuming this is if you have a 28 day cycle and you ovulate on day 14?? I've been reading up on this far too much and I'm thinking even if you did ovulate on day 14 but implantation took 12 days, you probably still wouldn't get a BFP on day 14, right?
Anyway the test was negative and I've done two more since (including this morning - day 36 - all negative). Some things I've read say to test after you expected to finish your period and some say wait as long as 10 days after a missed period so I don't know what to think now!
Currently have very light orangey-brownish discharge (wow, who knew I'd be talking about my secretions with a group of strangers on the internet 
) after about a week of lots of white discharge. Not sure if boobs feel weird or if I'm imagining it! I've been feeling a sort of pressure/light cramping in my lower abdomen and I can definitely smell all the things. Might be some imaginary/non-imaginary nausea too!
I did fall pregnant a few years ago but did not continue with the pregnancy :( but now I find myself comparing everything to that time. I did have a similar discharge but it took me about two or three weeks to realise I'd missed my period and to test - by which time the test came back as an immediate B F P. I don't know if I just need to leave it for a week or so this time and see what happens!
What's the latest you've had a negative test even though you later turned out to be pregnant?
Hope my ramblings make sense!
MG