Even though it wais winter now, the mountains were still full of life.

It's still winter

Firstly, that place hain the mountain region, there is a village that plantings camellias.

Even though it wais winter now, the mountains weare still full of life.

I changed “was” to “is” for consistency with “now.”

And I changed “were” to “are” for consistency with your use of present tense in the next few sentences.

Firstly, that place has a village whose residents have been planting camellias.

“A village planting camellias” doesn’t make sense to me, so I changed it so that the people of the village have been doing the planting.

If I were to translate your Chinese sentence, “先是那地方有个村子养茶花的,” I might go with something like, “First of all, camellias are cultivated in a village there.”

Secondly, there are many tea gardens on the top of (a mountain,) the main mountain we climbed yesterday.

It would read better if you deleted “a mountain.”

To me “tea gardens” sounds like decorative gardens where tea is served as a beverage. If these are commercial gardens growing tea as a crop, I would call them “tea plantations.”

And thirdly,: the streams along the trail.

This sentence is grammatically incomplete—it lacks a verb—but this seems OK to me in context.

I took a lot of pictures and videos during my hike / while hiking, but I want to show you the water in the streams first: https://www.youtube.com/shorts/8o8rtwBVoGM ¶
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I like the sound of running streams and I hope you like it too.

Even though it wais winter now, the mountains were still full of life.

I think there are three reasons for the lively feeling.it felt that way.

The way the word lively is typically used is more in the sense of "full of energy." This is subtle, but no one would say "three reasons for the lively feeling."

Firstly, that place has a village planting camellias.,

join this sentence with the next

Nowand it's camellia season, andso many camellia flowers are blooming between the mountains and the village.

Secondly, there are many tea gardens on the top of a mountain, the main mountain we climbed yesterday.,

You know,and tea plants are always green.

I took a lot of pictures and videos during my hiking, but I especially want to show you the water in the streams first: https://www.youtube.com/shorts/8o8rtwBVoGM ¶
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I like the sound of running streams, and I hope you like it too.

It was sunny, not too cold, but a little windy, and the scenery was fantastic.

I would combine this sentence and the next one. That way you're listing 3 things (sunny, windy, fantastic scenery) and it sounds more natural.

Even though it wa's winter now, the mountains were still full of life.

I think there are three reasons for theis lively feeling.

Firstly, that place has a village plantingwhere camellias are planted.

Now that it's camellia season, and many camellia flowers are blooming betweenthroughout the mountains and the village.

Secondly, there awere many tea gardens on the top of a mountain, the main mountain we climbed yesterday.

And thirdly, there were several beautiful streams along the trail.

"And thirdly, the streams along the trail" isn't a complete sentence. You don't have to write what I put, I just gave an example of how you could change it.