I own a WELLTECH (juke-box-style)10 CD Tuner HIFI

Hi.

LIDL electronics are usually purchased in big stocks and then re-tagged in Europe or in US, it will be impossible to find schematics.

however from your description I can tell you already that the problem is restricted to the optical drive. The fault is either lens , controller, or motor. Most likely the lens.

When lens pickup is dirty or defective the unit will spin the disk, but the drive will not be able to focus to the TOC on CD/DVD, and will skip to next disk.
To understand if it is the lens , all you need to do is observing if the disk is spinning. A test may be done jumpwiring and energizing directly the spindle motor.

If the spindle motor is working, then it is the lens, otherwise the motor.

If it is the lens, before replacing the lens you can try cleaning it with isopropyl alcohol and water (about 70% solution).

The controller board can also cause problems, but if the unit is operating this is rather uncommon.

I will restrict the diagnosis to lens or spindle motor.

Parts for no brand electronics are difficult to find, however both lens and spindle motor are quite standard parts.
The parts can be purchased at an electronic spare retailer, or if the part number is printed on motor, use part number to get motor specs on google and buy same part.

The lens pickups are almost all compatible on this kind of unit if you take one from a similar unit. Lens replacement is quite a delicate job, and there is a good chance of unsuccess because of lens alignment issues.