Free algebras via monoidal envelopes

For any morphism of $\mathrm{\infty }$-operads $\mathcal{P}\to \mathcal{O}$, we show that the free $\mathcal{O}$-algebra on a $\mathcal{P}$-algebra admits an explicit formula as the colimit over the $\mathcal{O}$-monoidal envelope of $\mathcal{P}$, providing a new and simple proof of the existence of relative free $\mathcal{O}$-algebras.