Learn Finnish: Hän sai magneetin irrotettua seinästä. - He got the magnet detached from the wall.

This is the core of the resultative achievement structure you are learning. When you use saada to mean managing to get something done, the action that was completed (the passive past participle) must be in the partitive case. So, irrotettu becomes irrotettua.

Why is the object magneetin in the genitive case?

The object in this saada construction follows normal object rules. Because the action of detaching it was successfully completed, it takes the total object form. For singular nouns like magneetti, this means using the genitive ending -n.

Where does the word irrotettua come from?

It is built from the transitive verb irrottaa (to detach or remove something). You first form the passive past participle irrotettu (detached), and then add the partitive ending -a to fit the saada structure.

Why does 'from the wall' use the elative case seinästä instead of the ablative seinältä?

While English often says 'off the wall', the verb irrottaa strongly pairs with the inward local case elative (-sta/-stä) to show separation and removing something from its attachment.