Sign in at /account/sign-in/ with your email address. Continue either sends you to your organization's OpenID provider, if one is connected for that domain, or asks for your password. There is no separate username.
Organizations can connect OpenID Connect from Settings → Single sign-on when the account email is on a domain they control. WebFinger on that domain must advertise the issuer. After that, anyone with an address on the domain can continue from the sign-in page and authenticate with the identity provider. /account/sign-in/openid/ is the same sign-in step for a domain that is already connected. Connecting a provider is owner-only and happens from Settings → Single sign-on, not from that public page.
If the identity provider has not verified the address, you can still use the account. A notification asks you to confirm your email until you do.
If it will not let you in
A failed sign-in always says the same thing, that the email or password is incorrect, whatever the actual reason. That is deliberate: a more specific message would tell anyone typing addresses into the form which ones have accounts here.
Sign-in attempts are rate limited per email address and per source address, so after several failures in a row you may be asked to wait.
Reset the password
/account/password-reset/ emails a link to the address you enter. The link expires.
Password reset does not clear two-factor
Proving you control the email address says nothing about the second factor. If you have lost your authenticator and your recovery codes, resetting the password is not enough. Open a ticket. See Two-factor authentication and passkeys.
Signed-in sessions
Settings → Sessions lists every device signed in to your account, and lets you sign out any of them. Sign everything out after using a machine you do not control, or after a password change you made because of a leak.
Your profile
Settings → Profile holds your name and company. Light or dark mode is under Settings → Appearance. The account email is the account identity, so changing it is a support request rather than a form. See Settings for the rest of the menu.