Sign APK

Generate a keystore

keytool -genkeypair -v -keystore whatever.keystore -alias whatever -keyalg RSA -keysize 2048 -validity 10000

Sign the APK

jarsigner.exe -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore whatever.keystore "C:\Users\user\Desktop\FGO\Fate Grand Order English_v2.31.1_apkpure.com\dist\Fate Grand Order English_v2.31.1_apkpure.com.apk" whatever