Once you reach Commander level 50, you will unlock Alliance Donations and Stark Tech. You can use gold or power cores daily to donate to your alliance, which will provide Alliance XP to level up your Alliance along with providing yourself with Alliance Credits.
You can then use your Alliance Credits for Stark Tech bonuses for your personal team and characters, improving their power based on their traits. The max amount of upgrades you can perform will depend on your Alliance level.
If you join a new Alliance, you must wait 24 hours before being able to make a donation to your new Alliance. You will retain your Stark Tech bonuses in full after switching alliances.