[1] I moved qemu01.jinja2 from device-types/ to device/

That's right has qemu01.jinja2 describe a device and not a device-type.

Q1: how I can remove/delete qemu01 device-type I added by mistake?

In the admin interface: http://localhost/admin/lava_scheduler_app/device/

[5] Now, the following command:
lava-server manage devices add --device-type qemu --worker
stretch.localdomain qemu01 <<================

qemu01 here is "hostname". Could you, please, explain this? It seems
to me that qemu01 (device type qemu) should be target, right???

The device "hostname" is just the name of the device.

Rémi Duraffort