Strength can be exhibited in many different ways, and I wouldn’t limit this to ”female protagonists” only. The world is filled with people with different lives and varied experiences, but everyone faces difficulties. Trauma, hardship, and so much more is a part of our human lives. We cannot escape the suffering, but how we confront hardships can exhibit strength or weakness. It has less to do with ”traits”, because each person is different and diverse characteristics can still underpin strength. More than anything, strength comes from mindsets. How do you face hardship? What will you do when obstacles are in your way? Which action will you take in the face of suffering? Having the right mindset will allow you to overcome no matter what personal traits you use to achieve that.
The fantasy genre has a tendency towards ”perfect” protagonists. They rarely make mistakes. If they do, those are obvious plot devices, rather than part of a character arc. As far as I’ve found, fantasy focuses more on plot than characters, although there are exceptions. George R.R. Martin is very good at showing what different characters will go through, and I think Sansa Stark is a great example to teach us about the inherent strength we have to overcome the horrors and hardships that life throws at us, as just one example among many. Guy Gavriel Kay, a grossly underappreciated fantasy author, has a true knack for showing the human capacity to exhibit strength in the face of hardship. He’s a real master at this (and has been publishing books since the 80s!).
With regards to modern fantasy, especially young adult fantasy, I personally believe we need to break away from this idea that ”female characters have to be strong”, that they have to be mini-action-figures who are perfect. No one is perfect. I personally hate reading stories with Mary Sue type characters. At the same time, there is a history of male characters in fantasy being portrayed as perfect too. I love to see characters, both male and female, overcome their weaknesses, face their fears, and become kickass, as opposed to them being kickass from the start.