How Wallet Mobile Softwares deal with Wallet Balance and Transaction history?

Question

I'm trying to build a mobile software Wallet made in Flutter with a Bitcoin Core Full node that communicates with a Node.js backend.

The problem I can't find the answer to is: how can wallet softwares keep up all the incoming transactions from potentially thousands of addresses?

I wouldn't really like to rely on any third party APIs like blockchain.com but in the other hand Bitcoin Core and bitcoin-cli don't allow me to check transactions starting from an addresses other than the ones from manually imported wallets (which would be clearly impractical from a software that is expected to work fast, with multiple HD wallets and mostly of all, where I don't have to store the private key other than the user device.).

So, how do wallets work in this regard? Do they rely on a third party and if so, how are those parties able to work as well as (for example) blockchain.com? What am I missing?

If the solution is to import into BC all the wallets created via Dart, is there a way to manage thousands of users in one or more Bitcoin Core nodes ensuring 100% security and speed?

Thank you to everyone that provides as many details as possible.

Answer 1

I was able to find an answer which for me was Electrum Server and similar (for example ElectrumX). I wasn't aware at first of the existence of softwares able to analyze blockchain and provide many informations like UTXOs and transaction history starting from data provided by the Bitcoin Node.

Here's the link for electrumX: https://github.com/spesmilo/electrumx

I want to link also this website that helped me a lot: https://bitcoin.design/

Please be aware that many software like this exist and I haven't tested extensively any of them. So please Do not trust, verify, always.