SIPC covers custody failure, not investment loss. If your broker collapses and your shares are missing, SIPC works to make you whole. If your shares simply fell 80%, SIPC is irrelevant.
Coverage is per customer per separate capacity, so an individual account and a joint account are counted separately. Futures positions and most FX and crypto holdings are outside the scheme, which is a common and expensive misunderstanding.
Example: a failed broker leaves you short $380,000 of stock and $90,000 of cash. Both fall within the limits, so the full $470,000 is covered. A customer with $900,000 of securities is covered to $500,000 and becomes a general creditor for the remaining $400,000.
Related: fscs-protection, customer-protection-rule, street-name, broker-dealer