Hi.
I found the how-to-backup with TWRP ,
But what if my current phone goes broke and i buy another brand of phone… Can i then use the backup on my new (other brand) phone that i made with this broken phone? Or does it need to be exactly the same device?
May sound as a stupid question but u Never know.
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
As @Manoj says in his HowTo TWRP makes a complete backup of your device and so it would probably not work on another phone unless it was the identical model.
I think the question you need to ask yourself is what do I need to save that is essential to the way I use my phone. If you mainly want your personal data - files, folders, etc. then it’s easy to copy this to a USB sick or your own computer. If you have an ecloud account and install /e/OS on another phone then you will find all this in your cloud and by using the sync feature it will be automatically restored to your new e-phone.
If you want to keep your list of contacts this can be easily exported as a vcf file and then reimported on a new phone. Calendar entries can be saved in the same way using ics files, but by using an online calendar such as the one in the e-system you can reconnect and sync calendar information on a new phone. Chats & IM exchanges are more problematic and depend on whether the app has a function for backups. Signal does, for example. But you have to copy & save this on another support yourself. Some apps let you save to well-known cloud operators like Dropbox but again this is specific to way the app was made.
On the other hand, if you want a sort of all-in-one tool that saves all apps and their data, then in my knowledge the only solution is to use an app like Titanium Backup which means having a rooted phone. But you will have to be careful restoring to a different brand of phone because many of the system apps are device-specific and Titanium warns you not to restore them.
The new Lineage 18 ROM has the SeedVault app which supposedly makes all this choice of backup & restore much easier but you will have to wait for it to be integrated into the /e/OS as I read that this is planned.
To sum up the problem, from my experience I would compare the backup & restore to another device operation to switching operating systems on a computer. Some programs are universal and exist on Macs, Windows or Linux systems and if you have the technical know-how you can get them to work with your data on whichever system, but there will always be surprises even between upgrade versions of the same system! And you’ll have to learn which are the user-friendly apps and which require a degree in computer science to figure out how they work. 
Thank u for ur advanced reply.
I have a nextcloud account but want a hardware backup to. Will a micro SD card in the phone also work?
For things like pictures , notes, keepass database , aegis database…
I rather want to ask it before buying this
Yes, any physical support that you have read/write access too will work. I’d suggest using Total Commander as it operates using two windows which you can swipe between and from the Home button you should see your micro SD card in the list. It may ask for permission to access it, but once given you’ll see an empty directory if it is a new card.
Swipe back to you home directory and choose Internal shared storage
which should be /storage/emulated/0
From there just tick on the directories you want to copy, then tap on the folder icon at the bottom (2nd from the left) and choose Copy.
The Aegis app, BTW has an export function. If you go into the settings scroll down to the last entry Export and it will put a json file at the root of your storage unless you choose another folder for it. This is all you need after a reinstall of Aegis to find all your registered sites for 2FA.
I also read the TWRP wiki, and through this thread, I thought I would ask a question related to the topic.
I have most things (contacts, calendar, photos) synced with my nextcloud instance, so my primary concern are all the things that make a phone “fully functional,” e.g. the apps: authenticators, garmin connect, etc. I have logins, backup recovery passwords, etc. So, I could just always reinstall everything again, and set them up, but it’s tiresome and annoying. Even if it only take 2-3 hours to fish up everything, and figure out what I need again.
I would like to be able to make a FULL image, with all data including apps (and their data), that I could retrieve whenever I needed to. Is there such a thing? I had used adb backup and restore in the past with great success, but that did not seem to work with /e/ for whatever reason.
The first time I had to restore from scratch it was VERY annoying, as I had to fish for the recovery information all over the place, esp. work related 2FA. The second time, it was better since I knew what I had to do, but it was still annoying. The third time, I assume when I reach it, it will go quicker to just re-install everything. But, there has to be a better solution??