How to build customer app on sharepoint

Wawan4137  it is possible, but as usual, it would be a complex solution.

For example, you can do a lot of stuff using PNP provisioning engine also using azure function + devops pipeline to get a solution to different sites/tenants.

For example:

Using PNP provisioning engine you can export sharepoint site with all webparts, using its config file you can write script to include/exclude things like navigation/security and etc. If you need some custom fields with value you could when use Sharepoint Site design script to add some default values to fields or do something with theme. With site design you can even call MS automate to do some cleanup or some additional steps. 

Microsoft working on some provisioning solutions as we speak but it is always complex project.

Good luck.