Thunderbird for Android, like other Android apps, normally uses the "Contacts" app that is built in to Android. But you may need to give Thunderbird permission to access that app. You may have skipped giving Thunderbird the permissions it needed to access your Contacts when you set it up.

I think there is also a checkbox in the Thunderbird settings that tells Thunderbird to use your Contacts. And I think if you check that off, then it will prompt you to grant Thunderbird the permissions it requires to do this.