Visitor visa from India

[ind2007]

Wife would like to call her mother for a visit and has filled in the form on visas-immigration.service.gov.uk.
Not yet paid for the appointment as she will be covering all the costs for flights and stay but it did not not ask for more details on this. Will this be requested once mother in law is invited to a biometrics appointment or will wife get the opportunity to upload proof of income, house, savings etc once the appointment has been booked?

Will the mother in law also have to take anything from her side to the appointment apart from passport and ID as she just lives at the sons house and is retired, so has no income with all costs there covered by her sons there.

Any help would be appreciated.