@nbjani there is no set structure to any questions, just suggestions. You're right in thinking the best way to approach competency questions is to STAR them, but that strategy has been advised based off how the question is structured, it invites you to talk about a time or an example where you've displayed that particular skill.
With strengths based questions more generally, it heavily depends on the way the question is asked. Let's look at two different ones that I've been asked in the past:
1. "What would your friends say are your top qualities"
2. "Do you prefer focusing on the bigger picture or the smaller details"
Although both questions are strengths based, I can only qualify the second with an example naturally. It would just be plain awkward if I provided an example of a time my friends complimented me (they do so very rarely). However, with the second, I can talk qualify why I think I prefer focusing on the smaller details, and leverage an experience that made me come to that realisation.
So, look at the way the question is structured, and check to see if it invites any examples. Where it does, be sure to use it as it will give you more to say and help sell who you are as a candidate. However, where it doesn't invite examples, don't stress too much, providing one is just a bonus anyway.