How do boosts work on alliance quests on Marvel Contest of the Champions?

I understand how linked nodes work. They are purple arrows where one end is the source of the boost, and the other end where the arrow is pointing to is the receiver of the boost.
In the image below we can see a droid boosting Ultron (the big boss). If someone kills the droid, the boost is removed making it much easier to defeat Ultron.
What I don't understand is those orange circles around bosses. For example, in the image below Ultron also has an orange boost. What are those boosts? Who is the source? And is it possible to remove them?
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How does alliance quest work MCOC?
Maps per QuestAlliance quest events last five days. Each individual map will last 24 hours. Once the game day is over, the system will calculate the amount of prestige your alliance has earned and, if eligible, it will move forward to the next difficulty level.Can you stack boosts in contest of champions?
Boost items provide temporary buffs to your Champions. While you can apply multiple boosts to your account, you will not be able to activate more than one boost of each category at a time. For example: Activating a Lesser Attack Boost for 10% means that you will not be able to add another attack boost.Tips and Tricks: Alliance Quests | Marvel Contest of Champions
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Images: Helena Jankovičová Kováčová, Nataliya Vaitkevich, Nataliya Vaitkevich, Nataliya Vaitkevich