Cross Platform App Development

I am looking for someone to help me with cross platform app development. I have a high-fidelity Figma design. You will need to use Figma mocks to build.

Scope is
1. Build a backend (database)
2. Build front end
3. About 30+ screens. All mocked in Figma.

Please send me the following
1. How much will it cost
2. Time to get this done
3. Example of backend and front end work for x-paltform app
4. Recommend which platform should we use.

Ask questions if you have any.