A proxy is a small, low-resolution copy of a large original video, used for browsing, searching and previewing so the multi-gigabyte original never has to stream. The final cut uses the original; everything before that uses the proxy.
A proxy is a lightweight copy of a big video file. A 4K creator clip might be 8 GB; its proxy is a 480p version a few hundred megabytes at most. You browse, search and preview the proxy. You only touch the original when you render the final cut.
Proxies are why a footage library can play any moment in a second, on any laptop, without downloading the original. It's also why storage and streaming cost stays sane at hundreds of clips per brand: the 8 GB original sits in storage; the proxy does the daily work.
Editors know proxies from Premiere and DaVinci, which create them locally. A library does the same thing once, centrally, so every editor on the team gets the same fast preview.
A brand uploads a 1.5 GB 4K clip. The library makes a 20 MB proxy, a poster image and an audio track, then transcribes and analyzes from those. The original is never downloaded again until an editor exports in full quality.
No. The proxy is only for finding and previewing. Exports and Premiere imports use the original file.
In BuildYourAds, 480p. Enough to judge a shot, small enough to stream instantly and store cheaply.