Are all adjectives determiners?
No, all adjectives can not be called determiners.
To understand the concept better, kindly refer to the explanation that follows and the statement provided by your friend Nilesh Sharma:
Although English grammar does not have determiners as a part of speech and usually considers them as adjectives, there can be some differences between the two.
We can use more than one adjective in phrase. However, we cannot use more than one determiner in a phrase. For example,
A new beautiful car (adjective)
A/ The/ Our car (determiner)
Adjectives have degree, whereas determiners do not. For example,
small, smaller, smallest
Determiners are compulsory but adjectives aren't. For example,
Many people here are vegetarian. (Determiner)