Connecting TV+DVD+VCR

Please specify the brands and references of the equipment.

For the Cube, connect it via HDMI, but that shouldn't be a problem. For the VCR, it needs to be connected to the SCART input on the LG.
You can start playback of the VCR and simultaneously record the source on the LG, meaning the SCART socket where the VCR is connected.
What I don't know is if your LG is connected via HDMI to the TV, can we see the video tapes. To make this backup, it might be simplest to temporarily connect the LG via SCART to the TV.
Insert the tape, at the beginning of the playback press pause, and simultaneously press "record" (LG) and play on the VCR. Once the copy is finished, burn it onto a DVD.
It's quite lengthy.
There are also multimedia hard drives with analog inputs, you could make your backups on a high-capacity hard drive, without going through a DVD, as the multimedia hard drives record in "ts" format, which would be a "video library." This format can be read by any PC. You could manage these backups by renaming the files via your PC and then playing them back on your TV.
In your case, if you have 100 tapes for example, you will need as many DVDs, and that's quite cumbersome. With a multimedia hard drive, everything would be on a medium or large capacity hard drive.
There are quite a few possibilities.
Another option, for commercially available VHS tapes, check if the DVD is available for sale used on sites like Priceminister, or at low cost on Amazon.fr or Cdiscount.
If needed, sell the tapes used on Ebay or Priceminister.

Thank you again.
I will respond to the second part of your message: what I want to save on DVD (how long (???) are shows that I'm unlikely to find in one form or another commercially...except maybe one day on I.N.A.
The duration of the operation doesn't bother me: I start the copy to the DVD hard drive and then I go to bed...the next evening I start the burning process in the same way after isolating the useful part on the hard drive!
I have done this before with a direct connection from the tape recorder to the DVD, but that is my question: I would like everything to be installed permanently without having to modify the wiring: all this equipment is built-in along with an amplifier and a turntable...that's a lot of wires! I will therefore follow your advice.
Please give me a few days to test it: what I want is for the installation to allow me, without fiddling with the wiring, either to watch TV and record it on the DVD or to transfer from the tape to the DVD.
In any case, thank you.

Try connecting the VCR via SCART to the LD input and set the LG to HDMI. Does it work or not? Otherwise, you'll need to connect the LG via SCART.

Phew, I managed it using what is probably an unorthodox method...! I connected the SCART cables from the decoder, the DVD, and the VCR to a triple SCART socket (found at the back of a cupboard) that is invisible but manually accessible. In normal operation (DVD + TV) I unplug the VCR. In tape to DVD copying mode, I unplug the decoder and plug in the VCR. The disadvantage of not seeing on the screen what I am recording doesn't bother me. Anyway, I check at the moment of burning the hard drive > DVD.
Thank you again for your cooperation.