Once you have the data files, you can import data into scratch Orgs or any other Orgs using the following commands:
sfdx force:data:tree:import --plan./data/Account-Contact-plan.json -u MyTPO
That --plan is used to provide the path of the plan JSON file.
Here, MyTPO is, again, the alias of the Org we want to import the dataset to.
Once you import the data, the command prompt will have the results and the IDs of the dataset.