It's KaiOS[1]. I think their biggest hit was the JioPhone in theIndian market[2] but the phone lineup is quite impressive for an alternative system.

They've got some funding from Google[3] as well.

[1]: https://www.kaiostech.com/

[2]: https://www.financialexpress.com/industry/technology/relianc...

[3]: https://www.theverge.com/2018/6/28/17513036/google-kaios-inv...