You can mostly backup everything but it's impossible to make a perfect backup like the old days anymore because of the TEE. Flashing a new ROM will change the keys and permanently make the old data worthless. Stuff like Google Authenticator for example simply won't backup even with a perfect bit copy.

Apps will restore okay but many will be logged out and have lost their permissions and push notification registration with Google.

If you rooted your phone you can use Neo Backup can't backup full ROM, but is great for backing up applications configured. For non root, I honestly don't know. There are other applications for backup but I used this one and is easy to use.

In your recovery there should be a backup option. Pick that and back up everything.

Then make sure you copy that backup off the phone in case you really hose things and have to start completely fresh.

no such thing exists. there's a buncha stuff claiming to do it but I've tried everything there is and everything fails differently, but consistently.

source: tried for close to two weeks to create a perfect clone to a similar phone (Poco F1 to Oneplus 6T - both have the same SoC, LineageOS, rooted) so that all apps and settings and storage and whathaveyous are copied and I failed. even the supposedly standard backup - wipe - restore, on the same phone, doesn't work.

afaik you can do a nandroid backup on twrp or just dump the raw partitions, but my android knowledge is dated and even before treble and a/b stuff so it might not work now.