A 16 character password with all four groups uses a 72 symbol alphabet. Entropy is 16 × log2(72), about 99 bits, when every character is drawn uniformly. The page leaves out 0, O, 1, I and l so a handwritten copy is easier to read.
The draw uses the server's cryptographic generator. The password is returned on this page and is not stored. More format options live in DevTools.
Questions
Is entropy the time needed to crack it?
No. It is the size of the draw. The time depends on how a site stores the password.
Why is 8 the minimum?
Below that, this page's alphabet is short for general use.
Can I generate another?
Each click draws a new password. The previous one is not stored.
Where is the larger tool?
In DevTools, at https://devtools.wipmob.com/tools/password, with analysis and other formats.