Out-of-stock products are ordered from the manufacturer but until they physically arrive they do not appear as available and cannot be purchased.
Until before the Covid 19 pandemic, products were available again in about 15 days, more or less, but this is not possible now in all cases.
For out-of-stock products you have a box where you can enter your e-mail address and when the product is back in stock you will receive a warning email, in case you are still interested in it.