Where to get The alias password while app signing in unity

You generally have two passwords you need. One is securing the keystore file. Think of it like an encrypted zip or rar file. The keystore password gives you access to the keystore. Inside a keystore file you can have one or multiple keys. Each of those keys have their own password. You need both when you want to build / sign something with a key.

Have a look at this screenshot:

The red password is the password for the keystore. The green password is the password for the key. When you create a new key you need to have the keystore password already entered. In the key creation dialog you just enter only the key password. The key is stored inside the keystore file. The key itself if protected by the key password and the keystore file is protected by the keystore password.