Hello,
I realized that the coordinates are not those usually used: when I create a cube and I do cube.position.x = 50 the cube moves away !
The coordinate system appears to be (z,y,x) instead of (x,y,z). Do you have an explanation ?
Thank you for your help !
Did you set camera.up?
I used :
const camera = new THREE.PerspectiveCamera(75, window.innerWidth/window.innerHeight, 0.1, 1000);
camera.position.set(300, 0, 0);
Must I set camera.up ?
A looong time after, I hadn’t received a notification for your answer 
Must I set camera.up ?
No, you wasn’t supposed to set it, unless you know you want to.
The mistake you have made is to set camera.position to (300, 0, 0), you probably want to set it to (0, 0, 300).
What is happening is that the camera is at (300, 0, 0) and looking to (0, 0, 0), so a cube in (50, 0, 0) is going to appear smaller than a cube at (250, 0, 0), for example.