While there are a lot of grammar and JLPT YouTube channels out there, many tend to use English as the medium of instruction. Sambon Juku, on the other hand, only operates in Japanese and really puts out some great content. For most of his videos, you will find bilingual subtitles (English and Japanese) which can be used to practice your listening comprehension. And as many in the comments section note, his Japanese is slow and easy to catch. He has a number of different types of videos but his most popular playlists seem to be the JLPT N3, N2, and N1 playlists where he breaks down some common grammar patterns seen at each respective level. He includes his own example sentences and does a great job of explaining the grammar points in simple Japanese.
Other video topics include explanations of honorific language (keigo), videos about the Japanese writing systems (hiragana, katakana, and kanji), lessons on casual Japanese, Japanese basics, and more. All in all, if you need something to supplement your JLPT or grammar studies, Sambon Juku's videos may be perfect.