Learn Spanish (Latin America): Sentí la madera crujir al subir.

With perception verbs like sentir, ver, and escuchar, using the infinitive means you perceived the action as a single, completed event or from start to finish. If you used the gerund crujiendo, it would emphasize that the action was ongoing or already in progress when you noticed it, translating closer to 'I felt the wood creaking'.

If I wanted to replace la madera with a pronoun to say 'I felt it creak', where does the pronoun go?

The direct object pronoun la would go right before the conjugated perception verb, giving you La sentí crujir. In these perception structures, the pronoun for the object you perceive goes before the main conjugated verb, not attached to the infinitive.

Why use al subir instead of a conjugated verb phrase like cuando subí?

Using al followed by an infinitive is a standard Spanish structure for saying 'upon doing something' or 'at the time of doing something'. You could perfectly well say cuando subí, but al subir sounds very natural, concise, and saves you from having to conjugate a second verb.

How do we know who is doing the 'going up' in al subir if the verb isn't conjugated?