What makes a password strong
Strength comes from unpredictability, measured in bits of entropy. A password chosen randomly from a pool of N characters with length L has L x log2(N) bits. A 16-character password using all 94 printable symbols has about 105 bits, far beyond what any attacker can brute-force. Length helps more than complexity: adding four characters typically adds more strength than adding symbols.
Humans are poor at randomness, which is why invented passwords follow predictable patterns. This generator uses the operating system's secure random source through the Web Crypto API, the same source used for encryption keys.
Security guidance
Use a different password for every account, store them in a reputable password manager and turn on multi-factor authentication wherever available. Never reuse a password that appeared in a data breach. When copying, be aware that your clipboard may be readable by other apps or synced across devices; paste the password promptly and copy something else afterwards if that is a concern.