The infinitive is used with falloir in a scenario you might be familiar with: when there are two verbs in a row. Specifically, when the first verb is a modal, which falloir can be.¹

The way I was taught falloir is that there are three basic structures.

Il faut + infinitive

Translation: it is necessary to; we (I, you) must

Example: « Il faut aller à l’école. » It’s necessary to go to school.

Il faut que + subject + subjunctive

Translation: (subject) must

Example: « Il faut que j’aille à l’école. » I have to go to school.

Il + indirect object + faut + direct object

Translation: (indirect object) needs (direct object)

Example: « Il me faut un nouveau rasoir. » I need a new razor.

The last one is pretty distinctive, but the first two have some overlap in meaning. So how do you decide when to use which?

The cases where it makes a difference include ones where you want to make it impersonal or collective (il faut + infinitive) on the one hand, or specify who must do something (il faut que + subject + subjunctive) on the other hand.

But when there is a pretty perfect overlap, you can simply choose which structure you prefer.

Impersonal / a general rule: « Pour cette recette, il faut verser l’eau avant d’ajouter les œufs. »

Personal: « Il faut que tu te dépêches, sinon tu vas rater ton vol ! »

Either: « Il faut partir très prochainement. / Il faut que nous partions très prochainement. »

_{¹ Personally, I don’t know how helpful this rule of thumb is,
since the passé composé and other tenses also consist of two verbs in a row.
That’s why I also gave the terminology modal.}