Custom URL, not display name
Steam has two names: the display name you change whenever you like, and the custom URL you set once in your profile settings. Only the second one is a web address, and it's what this code encodes. If you've never set one, your profile still lives at a long numeric /profiles/ URL — paste that through "Any link".
The useful placements are physical and social: a card at a LAN event, a sticker on a rig, a corner of a stream overlay, a tag on a second-hand controller listing. Anywhere someone would otherwise have to search your name among a thousand near-identical ones.
Worth remembering that profile privacy applies to scans too. A private profile shows almost nothing to a signed-out visitor, which is what most scanners are — set the profile to public if the code is meant to introduce you.
Everything happens in your browser. The QR encoder is built into this page, so your username is never sent to a server, nothing is logged, and there's no daily limit. The code is static — the link lives in the pattern itself, with no redirect service in the middle that can expire, start charging or go down.