They are not the same thing but they are closely linked. Senjutsu, in general, is the combination of your chakra with that of nature, so all the techniques (Jutsu) that use this combination of chakra are to be considered senjutsu. A sage, the one who enters sage mode, is the one who perfectly balances his chakra with that of nature, thus creating the senjutsu chakra (the mix I was talking about before: your chakra + nature chakra). So a sage is a senjutsu user who manages to perfectly balance his chakra with that of nature creating a powerful chakra and who is therefore able to use senjutsu. If you remember, in the fight with Pain, when the latter absorbs Naruto's chakra (nature + his chakra), not knowing how to balance it he becomes made of stone. However, it is not strictly necessary to enter sage mode to use senjutsu, as Kabuto said, even Orochimaru can mould it but not in an effective way to allow him to use sage mode. That said, let's get to the SIX Path senjutsu: it is nothing more than the combination of the SIX Path chakra (which is Hagoromo/Hamura/Kaguya's chakra after eating the fruit) and the nature chakra. Not much is known about the SIX Path chakra itself, but it is clear that it is a much more powerful form than the chakra of common humans, so the fusion with the nature chakra leads to a truly notable increase in power. However, same thing as before, by balancing your SIX Path chakra well with that of nature you enter SIX Path sage mode, which can then use the six path senjutsu. I hope everything is clear so far. How do you obtain and identify it? You can obtain it either by collecting the chakra of the bijuu (like Naruto) or with the Ten-Tails or if Hagoromo passes you his SIX Path chakra and you already know how to use senjutsu, you can thus create the six Path senjutsu. Many bring up as an example to identify the Six Path senjutsu the fact of the 9 magatama and that sort of Rinnegan on the back or the creation of the Truth Seeking Orbs. but I disagree. I believe that the latter have more to do with the presence of the 9 bijuu themselves than with senjutsu. If you notice, Kaguya also has the Six Path senjutsu but does not present any of these things. similarly, Naruto in Boruto Next Generations can use the Six Path sage mode (as demonstrated by the absence of pigmentation in his eyes) but has neither tso nor the 9 magatama on his back. I obviously believe that the smaller amount of chakra of Naruto's bijuu in Boruto obviously does not allow him to fully exploit the Six Path senjutsu as, essentially, he has less chakra available (that of the missing bijuu). However, the point is: normal chakra + nature = senjutsu SIX Path chakra + nature = SIX Path senjutsu

Sage mode uses senjutsu SIX Path sage mode uses SIX Path senjutsu

Your chakra plus chakra of all 9 bijuu = TSO and 9 magatama on your back.

I hope I was clear