This generator deliberately leaves out a handful of characters that are easy to misread: the uppercase set skips I and O, the lowercase set skips l, and the number set skips 0 and 1 — all specifically because they're visually near-identical to each other in many fonts (I, l, and 1 in particular). That trims the alphabet slightly, but it means a password copied onto paper, read aloud, or typed from memory is much less likely to fail because of a misread character, which matters more in practice than the small reduction in raw randomness.
With all four character sets enabled, the usable alphabet is 71 characters (24 uppercase + 24 lowercase + 8 digits + 15 symbols, after the exclusions above). At the default length of 16 characters, that's 71¹⁶ possible passwords — about 4.17 × 10²⁹, or roughly 98 bits of entropy. Each additional character multiplies the total possibilities by 71 again, which is why length increases security so much faster than adding more character types does: going from 16 to 20 characters (keeping the same 71-character alphabet) multiplies the keyspace by over 25 million, far more than switching on one more character class ever could on its own.
Longer is generally better — most security guidance now recommends at least 12–16 characters, which this tool supports up to 48.
Length matters more than complexity, but combining uppercase, lowercase, numbers, and symbols still meaningfully increases the space an attacker has to search.
No — passwords are generated locally in your browser using your device's secure random number generator, and nothing is sent or saved anywhere.