
You Developer plan license only covers work done in the Developer environments in your Tenant. If the other organization has Microsoft 365 E3 licenses then they do have access to Power Apps unless the admins have specifically disabled it. But to customize forms or build Canvas apps in their environment will require you to have an Office 365 license in their tenant. If you have an Office 365 license of your own you can use apps or forms built in their environment but you can't create them.