noun a person, place, thing, or idea. In this case, 'doorstep' is a noun referring to the area in front of someone's door.
pronoun a word that can function by itself as a noun phrase and that refers either to the participants in the discourse (e.g. I, you) or to someone or something mentioned elsewhere in the discourse (e.g. she, it). In this case, 'someone's' is a possessive pronoun
preposition a word that shows the relationship between a noun (or pronoun) and other words in a sentence. In this case, 'at' and 'on' are prepositions indicating location.
Refers to a property being located or available for purchase/sale at or on someone's doorstep.
Refers to a guest arriving at or on someone's doorstep for a visit or stay.
Refers to a product or service being promoted as being available at or on someone's doorstep.
Refers to a package or delivery being left at or on someone's doorstep.
Refers to a crime or suspicious activity happening at or on someone's doorstep.
A writer may use 'at/on someone's doorstep' in a metaphorical sense to describe a situation where something is very close or imminent, such as 'The deadline for the manuscript is on my doorstep.'
A psychologist may use 'at/on someone's doorstep' to refer to a situation where a person is facing a challenge or issue that is directly impacting them, such as 'The trauma from the accident is at her doorstep.'
A real estate agent may use 'at/on someone's doorstep' to describe a property that is located very close to another property or landmark, such as 'The new development is right on the doorstep of the shopping center.'
A journalist may use 'at/on someone's doorstep' to describe a situation where an event or issue is directly affecting a person or community, such as 'The scandal is on the doorstep of the mayor's office.'
A lawyer may use 'at/on someone's doorstep' to refer to a legal matter that is directly impacting a client, such as 'The lawsuit is at our client's doorstep.'