const array = [[{prop: 1},{prop: 2}],[{prop: 1}, {prop: 12}, {prop: 5}], [{prop: 6}]];
const newArray = [{prop: 1}, {prop: 1}];
const key = 'prop';
const val = 1;
const result = array.flatMap(n => n.filter(m => m[key] === val));
// или
const result = array.reduce((acc, n) => (
n.forEach(m => m[key] === val && acc.push(m)),
acc
), []);
// или
const result = [];
for (const n of [].concat(...array)) {
if (n[key] === val) {
result[result.length] = n;
}
}