When you create an Amplenote account using an third-party account such as Google or Apple, your Amplenote account is associated with the email address behind that account and with a random password. That means that it is possible bypass the third-party login and use your email/password combination to access Amplenote. Here's how to do that.
To add a password to your account, you will need to log off your account. For that, click on your profile name on the top left side and choose the option "Log out":
You'll be redirected to the landing page of Amplenote. Click on the "Log in" button on the top right and, on the login prompt, choose the option "Forgot password?" on the bottom right:
When prompted for the e-mail address, insert the one used for your Google account or Apple ID. This will send you a link to reset your password, which can be opened from any device within 24 hours.
Once it's set, your account is ready to use! You can now log in from any device or browser using your email/password combination..