I’ve read that Grignard reagants react pretty quickly with alcohols and carboxylic acids to form their respective conjugate bases. I’ve also learned that Grignards can be used for the prepararion of alcohols and carboxylic acids too. How is it possible to perform these reactions if the reagant would also react with the end product?
The addition of a Grignard to a carbonyl doesn't form an alcohol—not directly at least. It forms an alkoxide ion (i.e. the conjugate base of an alcohol).
Only when that reaction is complete, is water (or some other acid, ammonium chloride is used quite regularly) added to the solution. The water protonates the alkoxide ion to form the alcohol, and also destroys any remaining Grignard. This step is usually referred to as 'protic workup' or just 'workup'.
So the Grignard never sees any alcohol.
The same is true of the carboxylic acid formation (assume you're referring to Grignard addition to carbon dioxide). It doesn't form a carboxylic acid straight away, it forms a carboxylate ion.