Learn Spanish (Latin America): La grieta solía ser pequeña.

It comes from the verb soler, which means to "usually do" or "tend to do" something. In the imperfect tense, like solía here, it translates directly to the English "used to" do something.

Is soler always followed by an infinitive verb like ser?

Yes! Because soler acts as a helper verb to describe a habit or tendency, it is always followed by an infinitive. For example, solía ser (used to be), solíamos ir (we used to go), or in the present tense, suelen comer (they usually eat).

Could I just use the imperfect of ser and say La grieta era pequeña?

You could, and it would simply mean "The crack was small." However, using solía ser strongly emphasizes the contrast between the past and the present. It implies that the crack used to be small, but it definitely isn't small anymore!

How does solía fit in with the hacía... que past expressions from this lesson?

Both structures are used to talk about the past using the imperfect tense, but they do different jobs. You use hacía + [time] + que to say how long a past action had been going on. You use solía to talk about a past habit or state ("used to") without focusing on the exact amount of time it lasted.