Do I need both etc/webapi.xml and etc/schema.graphqls for one feature?

Only if you want both REST and GraphQL clients. The good pattern: build a Service Contract (Api/ + Api/Data/ + Repository) once, then expose it through whichever transport(s) you need. etc/webapi.xml maps an HTTP route + method to a service interface method — pure declarative. etc/schema.graphqls defines a GraphQL type and field, and a Resolver class translates the GraphQL args into a service-contract call. Both transports re-use the same Repository, so business logic lives in one place and you avoid the classic Magento 2 trap of REST and GraphQL drifting out of sync.