There are multiple approaches to achieve it depends upon How API's are configured & implemented:

API's are provided by module:

if you are using module's API:

for eg: jsonapi/node/{NODE_TYPE}/{NODE_ID}

Approach 1: In this override these specific API's and add title field of reference content field

Approach 2: You need to hit the API with provided ID of reference field, then from response of API, can get the title of reference field.

API's created by rest export view: can use following approach:

Add the relationship of content reference field in the view & add then add title field of reference content field

There are multiple approaches to achieve it depends upon How API's are configured & implemented (using Rest API):

API's are provided by module:

if you are using module's API:

for eg: /node/{nid}?format=json

Approach 1: In this override these specific API's and add title field of reference content field

Approach 2: You need to hit the API with provided ID of reference field, then from response of API, can get the title of reference field.

API's created by rest export view: can use following approach:

Add the relationship of content reference field in the view & add then add title field of reference content field