When 2 teams go to the final it means they are the best team among the other teams and basically both teams are at the same level, the only difference is experience. Indeed, based on the squad, Real Madrid looks better, but there is no guarantee that Real Madrid will definitely win, the Europa League final is an example when a strong team like Leverkusen is defeated so the same thing could happen in the Champions League, right?
In results, they are two best remaining teams and will compete with each other for the title. Though their quality can be the best and only among the best but their mentality strength is actually best.
I am strongly belief in strength of mentality of teams that already passed through many opponents along their ways to the final match. No doubt about Real Madrid's quality, mentality but Dortmund actually are not bad in quality and their mentality must be strong to win against strong teams like Atletico Madrid and PSG in knock out phases.
A closest comparison is Atalanta in Europa League and that team created a big shock by beating Leverkusen with a big difference in the final match last week. Dortmund don't need a big win and they will need to continue their defensive strength and try to score a winning goal, even only one goal to win the final.