“...But how does it have value, except for everyone involved believing it has value? ...”
Often that’s good enough! Remember the big circular stones on Yap Island that are sued as currency. Everyone there says they have “value”. Therefore they do!
https://en.wikipedia.org/wiki/Rai_stones
Like you this I have trouble with!
“...And who decided that Bitcoin miners would get rewarded for doing complex computerized calculations with more Bitcoin in their accounts; who exactly is paying the miners for their time and effort?
......”
How does this create value?
> How does this create value?
As a hard-to-counterfeit (probably a lot harder than paper bills) medium of exchange with a degree of scarcity.
Once those attributes had been established, speculation could take its course.