This tool is a frontend. The hard part — turning pixels into curves — is someone else's work, and it is worth saying whose.
Every trace on this site is produced by VTracer, an open-source raster-to-vector engine by VisionCortex. It is compiled to WebAssembly and runs inside your browser, from the fork at 1mak/vtracer.
MIT License
Copyright (c) 2020 VisionCortex Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
VTracer is dual-licensed MIT or Apache-2.0; the MIT terms are reproduced here.
Most online vectorizers upload your picture to a server you know nothing about, and go down when that server does. This one does the work in your own browser: no account, no upload, no history kept, and nothing to break but the page itself.