Learn Spanish (Latin America): Al encontrar el portal, lo desbloqueas.

When al is followed directly by an infinitive verb like encontrar, it forms a time-related phrase meaning "upon [doing something]" or "when [doing something]". It is a very common shortcut in Spanish to connect two consecutive actions.

Because encontrar is an infinitive, how do we know who is doing the finding?

We look at the second half of the sentence for the clue. The main conjugated verb is desbloqueas (you unlock). When you use the al + infinitive structure, the person doing that action is almost always the same as the person doing the main action. So here, it is clearly understood that it is you who finds the portal.

Is there a difference between saying al encontrar and cuando encuentras?

They mean almost exactly the same thing here. Using al + infinitive is just a slightly shorter, very natural-sounding alternative to using cuando (when) followed by a conjugated verb. It makes your Spanish sound fluid and native-like.

Why does lo come before the verb instead of attaching to the end of it?

The word lo is a direct object pronoun taking the place of el portal. As a quick refresher, object pronouns must go directly before a normal conjugated verb like desbloqueas. You are only allowed to attach them to the end of infinitives, gerunds (words ending in -ndo), or affirmative commands.