Upstairs AC from whatever type of equipment will provide some cooling air to the floor below because hot air rises and cold air sinks. Just as heating a lower floor provides some heat to a connected floor above. Whether it's enough or not depends on the air flow. I wouldn't think in either direction it would provide the same comfort as delivering the conditioned air where it's needed.

I've experienced a minisplit in a large open area, it works fine. As to cooling other rooms, again it's a question of airflow. Absent having a lot of large fans moving the air around, I'd not expect the same results as if each walled off individual room had its own source of conditioned air.

If you're looking to cover several rooms with a minisplit, the better answer is using ducted minisplit equipment where air is provided to each room.

I have a single 18k minisplit on my main level placed in a central room and it cools my 1000 sqft area very well. upstairs I have an large evaporator (18k) in the attic with ducts to each room plus the master bathroom and a return vent in the hall. Both the attic evaporator and main floor minisplit are connect to a single outdoor compressor unit. This time of year I can cool both floors well using only the upstairs evaporator. In the dog days of summer that does not suffice.

I installed a mini split in the upstairs of my 2000 sq ft home a few years ago. It is in a large open room used as the main bedroom. I seldom turn it on, as I like to sleep cold (but if the outside temps go into the 30's I'll use it to warm the room). We don't normally need air conditioning here (PNW) because the nights cool nicely, my room has walls of windows, so it sleeps like a screened porch almost. I live near the saltwater and benefit from cooling breezes after dark. My lower floor (the main living area), stays cool for the most part.
But a few years ago, the forest fire smoke choked the air and I couldn't open the windows to cool the upstairs in the summer, and it was stifling. So, I put in the mini split but didn't really use it for air conditioning because the following year we had no smoke, so I just kept the windows open...until last year when we had a record breaking heat wave with temps near 110 or over. Even the normally cool downstairs got sweltering, the upstairs unbearable. The two floors are connected by a small entryway/stairwell with a door at the top of the stairs and door at in the entry leading to the living room, kitchen/dining, which makes keeping the heating zones easily separated. I keep the doors to the bedrooms on the main floor closed always But last year when it was so hot, I ran the air upstairs and opened the doors between the floors. The mini split kept the upstairs cool, kept the downstairs not so cool, but cooler than without. I finally remembered I have portable fans, so I set those up downstairs to move the air and make it feel cooler.
This year, I added a mini split downstairs, just in case we have such suffocating heat in the future. I would not rely on the upstairs unit to cool the downstairs effectively, but it will likely provide some cooling as Elmer describes, when the hot air from downstairs rises and gets cooled. But it's not powerful enough to draw much air from downstairs via the fan system.