Allow an automation to add multiple values in one cell

Are you using Scripting in your Automation? The trick is to use the "Spread" operator on the previous array within the creation of a new array that contains additional elements.

Example below;

let myCarsModels = ["BMW", "Volvo", "Holden"];
let nextCar = "Ford"
let allCars = [nextCar, ...myCarsModels];

console.log(allCars)

Juggling arrays, you may also find the problem of having an array within an array - something like this - note how "Ford" is nested within its own Array element within the allCars array;

To correct this, you can use the flat() method;

let myCarsModels = ["BMW", "Volvo", "Holden"];
let nextCar = ["Ford"]
let allCars = [nextCar, ...myCarsModels].flat();

console.log(allCars)

Which will return the desired array of cars;

[ 'Ford', 'BMW', 'Volvo', 'Holden' ]

Similarly, an array of Airtable Linked Field values can be manipulated in the same way as the string data.

let myCarsModels = [
{ id: "recABC123", name: "BMW" },
{ id: "recDEF456", name: "Volvo" },
{ id: "recGHI789", name: "Holden" }
];
let nextCar = [{ id: "recJKL010", name: "Ford" }];

let allCars = [nextCar, ...myCarsModels].flat();

console.log(allCars);

We now have an array of car record objects;

[ { id: 'recJKL010', name: 'Ford' },
{ id: 'recABC123', name: 'BMW' },
{ id: 'recDEF456', name: 'Volvo' },
{ id: 'recGHI789', name: 'Holden' } ]