Voluntary Pre-Kindergarten?

Can someone explain VPK to me, specifically in Florida? I’m honestly confused. Online it says VPK is free to all 4 year olds. But I see you have to apply and qualify. So it is income based or first come first served? It won’t even let me apply for the 2020 school year but my son turns 4 in July. So do I have to wait until July to even apply?

And if I don’t get in, then what? Do I have to pay for a private Pre-K? Or do you just not put kids in school until kindergarten?

I’m just really thrown off because the public schools around here have pre-k but I don’t get the difference between that or VPK, if there’s any at all. I feel like I ask and am given “resources” that all say the same things but I’m not even understanding the basis of it all. I think pre-k would benefit my son greatly since I stay home with him and he hasn’t had much experience with socialization and being away from me for long periods.