Mead fermentation

Does mead has to be fermented in a glass carboy for primary, or is OK to use the plastic bucket just for primary fermentation, every video I have seen is done in the glass carboy...

Plastic bucket is fine for most occasions..

Only if you're fermenting at cool temps and expect the fermentation to carry on for a few weeks, would you really worry about alcoholic fermentation in a carboy

You would still want to transfer to a carboy when the activity starts to fall off and the SG is getting around 1.020 - 1.010, because sometimes meads will ferment normally for the first 2/3 of the process and take forever for the last part to finish. If thats the case, you'd want to get a good deal of the sediment from the bottom so that there's enough yeast to finish the fermentation. Then when your SG is the same for 3 days, rack off the sediment so it doesnt make off flavors

You can use either or!

I have done both. If you know/have a good idea that you are going to leave it in primary for a while it might be easiest and/or safer to go straight to the carboy. My best example of this would be a traditional mead; I started one a week before Christmas and went straight into the carboy, because I knew that I wouldn't rack it for at least 4-6 weeks. Melomels with fruit in the primary might be best to start in the bucket since you might want to remove the fruit after only 2-3 weeks and it would also be easier to remove it. All that said, I use Better Bottles, so to your question I am technically always using plastic.