Hi,

Create a new BoM or edit an existing one by going to Manufacturing ‣ Products ‣ Bills of Materials. Then, click Edit. The Apply on Variants option to assign components to specific product variants on the BoM is available once the Variants setting is activated from the Inventory application. If the Apply on Variants field is not immediately visible, activate it from the additional options menu.

And you can refer to this blog for more.
https://www.cybrosys.com/blog/how-to-manage-bill-of-materials-product-variants-semi-finished-products

Hope it helps.