Learn Italian: Gli stivali pesano molto, ma sono sia caldi sia comodi. - The boots weigh a lot, but they are both warm and comfortable.

You are right that sia is the present subjunctive of essere (to be). However, in this context, it acts as a fixed conjunction. When paired as sia ... sia, the phrase means 'both ... and'.

Can I say sia caldi che comodi instead?

Yes, absolutely! sia ... sia and sia ... che are entirely interchangeable in modern Italian. Both structures mean 'both ... and'.

Since gli stivali is plural, shouldn't it be molti instead of molto?

In this sentence, molto is an adverb meaning 'a lot' and modifies the verb pesano (they weigh). Adverbs never change their endings. If it were an adjective describing a noun (like 'many boots'), then it would change to match the noun: molti stivali.

Why do caldi and comodi both end in -i?

They are adjectives describing gli stivali (the boots). Because stivali is a masculine plural noun, the adjectives must take the masculine plural ending -i.