Each producer sets their own terms, so the answer to this question will be different each time, and we recommend you read the contract in full before completing your purchase. This can be read during the checkout process, before payment - please see a guide to previewing terms here.
However, here's a few general guidelines for using your beat.
If you purchase a license and create a song, you can upload the finished song to all streaming platforms, including YouTube, iTunes, Spotify, etc. Depending on the lease you purchased, there may be limits to this (e.g. number of streams)
You cannot upload the beat as it is, on its own, to any streaming platforms. You must add vocals and create a song out of the instrumental, unless you are using it for backing music for a video on YouTube (e.g. gaming, tutorials, vlogs, etc.). You cannot resell the instrumental as it is.
You may need to credit the producer (this will be expressed in the contract). This can be done in the title, description, or the visuals of your video - check with the seller if you're unsure how to credit them, or if it's required.
You may want to monetize your song on YouTube etc. Usually there should be no problem with this if you've purchased a license, but always check with the producer beforehand, as it's likely they will be monetizing it also, and will need to exclude your song from their claims.
If you download a beat for free you will be very limited in how you can use it. E.g. the producer will likely monetize any video you upload, you will only receive a tagged version, you will not be able to earn a profit from its usage, etc.