but when does allColumns is assigned to columns?

In the remaining case, i.e. where both isUser1Readable and isUser2Readable are true.

The chained ternary expression can be interpreted as an if ... else if ... else sequence:

let columns;
if (!isUser1Readable) {
    columns = allColumns.filter(column => !columnIdsUser1.includes(column.id));
} else if (!isUser2Readable) {
    columns = allColumns.filter(column => !columnIdsUser2.includes(column.id));
} else {
    columns = allColumns;
}

The conditional operator¹ is greedy. The expression is clearer with indentation:

const columns = !isUser1Readable
    ? allColumns.filter(column => !columnIdsUser1.includes(column.id))
    : !isUser2Readable
        ? allColumns.filter(column => !columnIdsUser2.includes(column.id))
        : allColumns;

• If !isUser1Readable is true, columns is assigned the result of allColumns.filter(column => !columnIdsUser1.includes(column.id)).
• Otherwise
  • If !isUser2Readable is true, columns is assigned the reuslt of of allColumns.filter(column => !columnIdsUser2.includes(column.id))
  • Otherwise, columns is assigned allColumns.

but when does allColumns is assigned to columns?

When both !isUser2Readable and !isUser2Readable are false.

¹ Although people frequently call it "the ternary operator," it's actually just a ternary operator (an operator accepting three operands). It's currently JavaScript's only ternary operator, but that could change. It's name is "the conditional operator."

As the ternary operator is define like this

condition ? exprIfTrue : exprIfFalse

1. Condition: is the condition which is checked
2. exprIfTrue: is the code which is process in case the Condition is evaluate to true
3. exprIfFalse: is the code which is process in case the Condition is evaludate to false

The code which you provide can be refactor like this

let allColumns = [];
if(!isUser1Readable) {
    allColumns = allColumns.filter(column => {
        return !columnIdsUser1.includes(column.id);
    });
} else if (!isUser2Readable) {
    allColumns = allColumns.filter(column => {
        return !columnIdsUser2.includes(column.id);
    });
}else {
    columns = allColumns;
}

You can learn more about Conditional (ternary) operator