From this page of the Hitman 3 game guide, you will learn how to change disguises to infiltrate the restricted areas of the map.
While carrying out missions, Agent 47 will have to play various roles: private detective, bodyguard, cook, waiter, photographer, and many more. In Hitman 3, there are two main ways to get a credible disguise without raising suspicion.
Taking the NPC outfit
Overpowering your opponent and taking their belongings is the first of the main ways to get a disguise in Hitman 3. However, it should be noted that on the highest difficulty level (Master), bloodily eliminating enemies ruins the disguise.
Of course, you can overpower your opponent first, hide him in a closet or container, and then change disguises. The end result will be the same.
Disguises left on the map
In some places on the map, you'll find abandoned disguises that Agent 47 can immediately change into. Taking advantage of such an opportunity opens the way to infiltrate restricted areas of the map and get better cover.