Polkadot.js browser extension does one thing: it manages accounts and allows the signing of transactions with those accounts. It does not inject providers for use by dapps at this early point, nor does it perform wallet functions, e.g send funds.
However, if you want to use any substrate based dApp, you need to download and install this extension even if you already created Polkadot account in other wallets or in Polkadot.js web portal directly (don’t mix up web portal with browser extension, it’s a two totally different things).
You can download the appropriate extension for your browser here:
This extension is an open source and the code is available on GitHub, so you can review and build it by yourself here if you want:
A browser-based vault for management of your account keys.
Once that’s done and installed, find the new icon for the extension as it appears in your browser. Note: If you can’t see your extension in Chrome, just click on this puzzle piece icon in the upper right corner.
Then click on the “pin” symbol, located to the right of the polkadot.js icon.
Now the extension should be visible in the browser.
Click on the extension symbol. You’ll see a pop-up window, where you can create your first account.
Be sure to write down your wallet's seed phrase and/or private key, keeping it in a safe place. This mnemonic key is what you need to restore your wallet if you change computers or want to access the same wallet from another device. It’s important that you protect it, or else you risk losing access to your assets.
Come up with an account name and password.