Move all of your collections from different dbs to a specific db on MongoDB using Node.JS! (based on this stackoverflow answer (https://stackoverflow.com/a/54715607/14614326))

move_dbs_to_one.js

// don't forget to init npm first
// npm init -y
// npm install mongodb

const { MongoClient } = require('mongodb');

// Replace the uri string with your MongoDB deployment's connection string.
const uri =
	'mongodb://[username:password@]host1[:port1][,...hostN[:portN]][/[defaultauthdb][?options]]';

const client = new MongoClient(uri, { options: 'here' });

async function run() {
	try {
		await client.connect();

		async function moveDocuments(sourceCollection, targetCollection, filter) {
			const sourceDocs = await sourceCollection.find(filter);

			console.log(
				`Moving ${await sourceDocs.count()} documents from ${
					sourceCollection.collectionName
				} to ${targetCollection.collectionName}`
			);

			const idsOfCopiedDocs = await insertDocuments(
				targetCollection,
				sourceDocs
			);

			const targetDocs = await targetCollection.find({
				_id: { $in: idsOfCopiedDocs },
			});
			await deleteDocuments(sourceCollection, targetDocs);

			console.log('Done!');
		}

		async function insertDocuments(collection, documents) {
			const insertedIds = [];
			const bulkWrites = [];

			await documents.forEach((doc) => {
				const { _id } = doc;

				insertedIds.push(_id);
				bulkWrites.push({
					replaceOne: {
						filter: { _id },
						replacement: doc,
						upsert: true,
					},
				});
			});

			if (bulkWrites.length)
				await collection.bulkWrite(bulkWrites, { ordered: false });

			return insertedIds;
		}

		async function deleteDocuments(collection, documents) {
			const bulkWrites = [];

			await documents.forEach(({ _id }) => {
				bulkWrites.push({
					deleteOne: {
						filter: { _id },
					},
				});
			});

			if (bulkWrites.length)
				await collection.bulkWrite(bulkWrites, { ordered: false });
		}

		const dataDB = client.db('Target DB'); // change str to the transferor db name
		let datadbColNames = await dataDB.listCollections().toArray();
		datadbColNames = datadbColNames.map((x) => x.name);
		for (dbName of ['Transferee DB 1', 'Transferee DB 2']) {
			let db = client.db(dbName);
			let dbColNames = await db.listCollections().toArray();
			for (colName of dbColNames.map((x) => x.name)) {
				let sourceCol = db.collection(`${colName}`);

				if (!datadbColNames.includes(`${colName}`)) {
					// make collection if it's not on target db
					await dataDB.createCollection(`${colName}`);
				}

				let targetCol = dataDB.collection(`${colName}`);

				// change the last param if you only want specific posts to be transfered
				await moveDocuments(sourceCol, targetCol, {});
			}
		}
	} finally {
		// Ensures that the client will close when you finish/error
		await client.close();
	}
}
run().catch((err) => {
	console.log(err);
});