Font awesome unicode some icons not displaying - unicode

Question

I am using unicode as per cheetsheet: https://fontawesome.com/v5/cheatsheet/free/solid
I tried changing font-weight as per answer here but it hasnt helped: Font awesome 5 on pseudo elements
Maybe the issue is in css I linked? I dont know where to get other css version if there is newer.
.icon {
font-family: 'Font Awesome 5 Free', 'Font Awesome 5 Regular', 'Font Awesome 5 Brands', 'Arial';
font-size: 18px;
font-weight: 900;
}
<link href="https://use.fontawesome.com/releases/v5.7.2/css/all.css" rel="stylesheet" />
<div><span class="icon brand"></span>airbnb</div>
<div><span class="icon"></span>amazon-pay</div>
<div><span class="icon"></span>bacteria</div>

Answer 1

The Unicode for the first and last icons are incorrect if you are using just Font Awesome 5.
 doesn't exist in any icon library or in Unicode at all.
 matches with glyphicon-facetime-video from Bootstrap Icons and replay_10 from Google Material Icons. The reason why it didn't show up as either is because Bootstrap Icons needs the class glyphicon and Google Material needs the class material-icons.
Moreover Font Awesome also needs a class assigned to it's element. The CSS class in OP code, .icon is sufficient (it has the FA font-family property), but if you don't have that, the built-in classes are adequate as well (free versions are: fa, fas, and fab).
HTML
With Unicode
<i class="fas"></i>
Without Unicode
<i class="fas fa-biohazard"></i>
:root,
body {
font: 2ch/1 'Segoe UI'
}
.fab,
.fas {
margin: 2px 3px;
font-family: 'Font Awesome 5 Free', 'Font Awesome 5 Regular', 'Font Awesome 5 Brands', 'Arial';
font-size: 1.5rem;
font-weight: 900;
}
<link href="https://use.fontawesome.com/releases/v5.7.2/css/all.css" rel="stylesheet">
<div><i class="fas fa-bed"></i> Airbnb Locator</div>
<div><i class="fab"></i> Amazon Pay</div>
<div><i class="fas fa-biohazard"></i> COV-ID Status</div>

Answer 2

Firstly, you need to use the link of FontAwesome either CDN in index.html(main file) or import css in style.css
cdn: <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.13.0/css/all.css">
import in css file: #import url("https://use.fontawesome.com/releases/v5.13.0/css/all.css");
eg li::before
selector::before{
content: "\f007";
font-family: "Font Awesome 5 Free";
}