Yeah like I say it can be a little confusing but you’ll soon get your head around this game’s little quirks. It was only a few months ago I was asking very similar questions on this forum.
Here’s a handy link:
https://ddm999.github.io/gt7info/
It tells you what’s currently in each of the dealers and how long for, with that you can make a note of any cars you might miss and save up for them coming round next time.
As already mentioned the general rule is they come in for about a week, sometimes two, go ‘out of stock’ and then come back again roughly three months later. A couple exceptions are three cars that’re on a 6 week rotation which are needed for the platinum trophy, and then there’s the ’Special Picks’. I’m not entirely sure how this works myself, but it seems to be each week one or two cars are chosen at random to come out of the regular rotation, and the last day for them is always Sunday. If there are any new legendary cars released as part of an update they tend to come up as special picks for a week and then go into the three month rotation after.
Hope that clears things up!