They mean the same. ‘Have said’ is slightly more imprecise in relation to time. You said this at some unspecified time in the past. ‘Said’ gives the impression of nearer in time to when you are speaking. ‘Just said’ would be only moments ago.

Both questions are making in past time one is in perfect past time tense. The other one is in simple past tense. The difference is about all long ,time ago long. Next is about short daily time words happen.

What Racky said. The first one is in the present perfect tense and the second is in the simple past, if you want to look into them further.