# app/controllers/landing_controller.rb
def index
    @data = 123
    render locals: { data: @data }
end

# app/views/layouts/application.html.erb
<%= render 'layouts/header', locals: { data: @data } %>
<%= yield %> (вызывается index.html.erb)

# app/views/layouts/_header.html.erb
<%= data =>