Bpmn modeler that work with flowable

I need to apply a modeler in order to work with flowable engine, can I use the Bpmn.io modeler for structure workflows and send it to flowable engine? or can I use another one?

Flowable can import models created in any BPMN 2.0 compatible modeler.
But keep in mind, the models created in such modelers are only useful for basic structure.
Engine specific features such as service task implementation, form assignment for user tasks and so forth cannot be modeled in these generic tools.
You are better off using the Flowable modeler if you are looking for an executable model.

Can we use the flowable modeler in a web application? or Flowable has a separate modeler similar to bpmn.io in order to check how it works?

The Flowable modeler can be (and has been) embedded in a web application.
Check out how it is embedded in the flowable-task app here:
flowable-engine/modules/flowable-ui/flowable-ui-task-frontend at flowable-6.7.2 · flowable/flowable-engine · GitHub

Note that this is the 6.7.2 tag as the flowable-ui frontend has been removed in 6.8