Jiyomi is made by one person. The site shows the same database as the app, with the same display rules.
The data comes from two files of the Electronic Dictionary Research and Development Group: JMdict (common words, CC BY-SA 4.0) and JMnedict (proper names, CC BY-SA 3.0, © Jim Breen & the EDRDG). Jiyomi’s database is a derivative work, distributed under the same licences — details and download.