It is a very comprehensive topic in itself and connot be condensed to one reply. At the same time, Hadith classification and categorization involves alot of subjectivity at the end of different Muhaddeseen as well. Following link is really helpful to grasp the concept of Hadith Grading:

https://www.islamic-awareness.org/hadith/ulum/atit.html

Please also see:

Discussion 59270

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Just to give a little gist of traditional view in this regard, Several weak ahadith may mutually support each other to the level of Hasan.

According to the definitions of al-Tirmidhi and Ibn al-Salah, a number of similar weak ahadith on a particular issue can be raised to the degree of Hasan if the weakness found in their reporters is of a mild nature. Such a hadith is known as Hasan li ghairihi (Hasan due to others), to distinguish it from the other type which is Hasan li dhatihi (Hasan in itself). Similarly, several Hasan ahadith on the same subject may make the hadith Sahih li ghairihi, to be distinguished from the other category Sahih li dhatihi.

However, in case the weakness is severe (e.g., the reporter is accused of lying or the hadith is itself Shadhdh), such very weak ahadith will not support each other and will remain weak. For example, the well-known hadith, “He who preserves forty ahadith for my Ummah will be raised by Allah on the Day of Resurrection among the men of understanding“, has been declared to be Daif by most of the traditionists, although it is reported through several routes.

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Ghamidi Sahab has a slightly different view in this regard.

For point of view of Ghamidi Sahab, please refer to the video below from 31:00 to 42:30

https://youtu.be/vknAZgYTnKw?t=1860