Both alcohols and carboxylic acids undergo substitution, but the processes are very different. Explain

Carboxylic acid show nucleophilic substitution because carboxylic acid contains a carbonyl group, and the carbonyl carbon is electron deficient as the electron density is shifted to the carbonyl oxygen so that nucleophile, which is an electron-rich attack on carbonyl carbon and shows nucleophilic substitution a shown:

Nucleophilic substitution

Alcohol show electrophilic substitution react as they contain hydroxyl group and the oxygen atom has lone pair that is electron-rich, so they respond with electron-deficient species called electrophile and form ether if the electrophile is strong reaction followed by intermediate if electrophile is present, then reaction followed by alkoxide ion.

Alcohol electrophilic substitution reaction