How to add an attribute to the Search?
In order to make an attribute which is assigned to the products searchable, please, follow the next steps:
1) Kindly navigate to Stores > Attributes > Product and choose the attribute required.
2) In the attribute settings, please, switch to the Storefront Properties tab and set the Use in Search = Yes option.
3) After that, set the Search Weight configuration.
Please, note that for the Weight field, 1 is considered the lowest search weight quality while 10 is the highest one.