In short
Your device is your key. There is no password to remember, and none to steal. One thing, set up once, means that losing the device is a small problem rather than a large one. It takes about a minute.
Keep a copy where you can reach it without this account. This is the one page you cannot look up from a phone you no longer have. Save it or print it now.
Set this up once
Generate your Recovery Mark
In the App, open Settings, then Security, then Recovery options, then Recovery Mark. You get a picture to print. Print it, and put it where you keep things that matter — a folder at home, a safe, a wallet card. Scanning it proves the account is yours.
That is everything. It cannot be set up later from a phone you no longer have, so it is worth doing now rather than remembering later.
If your device is lost or stolen
Your account is still there. On the replacement device, open the App and choose Recover my account.
You are back in, usually within a few minutes. If you never set one up, recovery still works — a person reviews the request instead, and you are asked to watch your email for up to one business day.
Once you are back in, open your devices list and sign out the device you lost. Signing back in on a new device does not do that for you, and until you do, the old one still works.
More detail
Everything above is the whole of what most people need. If you want the full picture — how the checks work, what each message means, and what to do when a step does not go as expected — see Account recovery in detail.