Does Saarbrücken means the bridges of Saar?
It looks like it, but originally it was just "the bridge over the river Saar" -- singular (Sarabruca, Sarebrucca, Sarebrugge).
The -n came later, from common use in constructions such as bi der Sarebrücken (modern, bei der Saarbrücke "by the Saar bridge") which used the dative case.
If you touch and hold the 'u', it should bring up other accented types of 'u'. Same with the 'a', 'e' and 's' (for the ß)
Oh sorry, I didn't realize you were talking about the mobile app. I've never used it, so I don't know for sure, but there are often ways to type modifications of existing letters by holding down a letter on the keyboard and waiting for options to pop up. So in this case you would hold your finger on the U key and wait to see if umlauts pop up as an option. Maybe you could try that?
Why is a bridge feminine?
Bridges are objects, not words.
And "bridge" is an English word, and English nouns do not have grammatical gender.
The word Brücke is feminine.
You might think I'm splitting hairs, but it's important to realise the difference between a word and the thing it stands for; and between a concept and a German word.
Grammatical gender attaches to a specific German word, not to a general concept. For example, "the potato" is die Kartoffel (feminine) in Germany but der Erdapfel (masculine) in Austria. The gender belongs to the specific word, not to the concept "potato".
And das Mädchen (the word) is neuter, even though a girl is female. The word is not the same as the thing it stands for.
Now, to answer the question I think you wanted to ask: there's no reason for why the German word Brücke is feminine. Grammatical gender is essentially arbitrary.
And it's not even uniform across languages that have gender; French pont "bridge" and Slovak most "bridge" are both masculine, for example.