Are you interested in how to create this setup with a script instead of adding all the nodes by hand? Then remove the HBoxContainers and all of their children and use the following code instead
func _ready() -> void:
const ROWS = 3
const COLS = 20
var root := $VBoxContainer
for row in range(ROWS):
var row_container := HBoxContainer.new()
root.add_child(row_container)
for col in range(COLS):
var cell := TextureRect.new()
cell.texture = preload("res://texture.svg")
row_container.add_child(cell)