Does natural exposure to the target language (without a formal teaching of grammar rules) guarantee L2 leaners to acquire the lanuage?

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Research suggests that much depends on the capability of the learner to abstract metalinguistic concepts in L2 and in L1. It also suggests that natural exposure does not guarantee acquisition beyond basic linguistic skills even in L1.https://ore.exeter.ac.uk/repository/handle/10871/16005

L2 acquisition and the level of attainment depends on a great variety of factors but yes, exposure to the target language will result in acquisition, at least passive. Language acquisition is a unique human ability that permits finding out the "grammar rules" from the input and add them little by little to one's interlanguage.

Exposure to L2 is never as intensive as was exposure to L1. In addition, the learner tends to interpret L2 in terms of L1 grammar. For both reasons it is necessary to teach formal L2 grammar. A very important factor is the age of the learner, too.