Sleep changes perminantly at 3-4 months old YepYep1. It's not a phase, it won't go back to how it was.
Newborn sleep, when baby us in the fourth trimester, mimics the sleep baby did in the womb. Sleep is passive. As long as all needs are met, baby will sleep and will only wake when a need isn't met (usually hunger comfort).
Out of the newborn phase sleep becomes active - getting to sleep how has to be worked at and is no longer passive. It also develop a into sleep cycles, with brief lighter sleeping stages every cycle.
This will last as long as it takes you and baby to find a way to help baby get to sleep. Some can do that quickly and so slerp soon improves. Some never find a way to help baby sleep that they are happy with and so on the future "train" baby.
Some things that are known to help babies get to sleep:
something to suck
Dummy, thumb, breastfeeding to sleep
movement
Bouncy chair, pushchair, car journey, rocking in arms
secure feeling
Being held while sleeping, swaddle or tightly tucked in, sleepyhead mattress, sling, parental closeness, eye contact while going to sleep, hand on chest, shushing, patting.