You can see every app linked to your Google Account on one page, then cut off the ones you no longer use. Stopping Sign in with Google for an app and removing an app's access to your Google Account are separate actions, so check what each app has.
Open the linked apps page
Google's help page on linked apps tells you to start from your Google Account's linked apps page. A Scan My Shadow guide gives this direct address:
myaccount.google.com/linkedapps
Sign in with the Google account you want to clean up. If you use more than one Google account, repeat this for each.
The same Scan My Shadow guide groups connections into three types: Sign in with Google, Google Account linking, and Access to your Google Account. The sections below cover two of them.
Stop using Sign in with Google for an app
This is the link that lets you log in to a site or app with your Google account instead of a password. According to the Scan My Shadow guide, you open the app's entry, select See details, then select Stop using Sign in with Google.
Before you remove it: if you still want the account on that service, make sure you have another way to log in there, such as a password set inside the app. If you want to be rid of the service entirely, delete your account there first, while you can still get in.
Remove an app's access to your Google Account
This is the permission that lets an app read or act on your Google data, such as your Gmail. Per the Scan My Shadow guide, you select the app, review what access it has, then select Remove access.
The review is the useful part. An app you signed in to once to try out may be holding access you would never grant today.
Removing access may not delete your data
According to Scan My Shadow, removing access stops the app's future access but does not delete data the app already collected, so it advises also deleting your account on the app's own side. Treat it as a sensible precaution: if an app held your email or files and you are done with it, use its own account deletion option too.
Old guides use menu names that no longer match
If you followed another tutorial and could not find the buttons, it may be out of date. An older xFanatical guide describes going to Security, then "Third-party apps & services", then "See all connections", then deleting the connection. The more recent Scan My Shadow guide instead starts from the linked apps page and uses the labels Remove access and Stop using Sign in with Google. If the menus differ from both, go by Google's own help page.
What this guide does not cover
If your account is managed by an employer or school and the options are missing, ask your administrator.
While you are reviewing account security, it is also worth knowing what happens to your Google passkey if you lose your phone.
