Learn Portuguese (Portugal): Este chá é muito amargo.

Great observation! Using é (from the verb ser) means we are describing the inherent nature of this tea—it is a type of tea that is naturally bitter. If you wanted to say that this specific cup of tea was brewed badly and is unexpectedly bitter right now, you would indeed use está (from estar).

Why use Este and not Esse?

Este means "this" and is used for objects physically close to the speaker (like the cup of tea you are currently holding or looking at). If the tea were closer to the person you are talking to, you would use esse ("that tea by you").

If the noun were feminine, would muito change to muita?

No. In this sentence, muito means "very" and is acting as an adverb to describe how bitter (amargo) the tea is. Adverbs never change their endings. Muito only changes to match gender and number (like muita or muitos) when it means "much" or "many" and describes a noun.

How do you pronounce the 's' in Este in European Portuguese?

In Portugal, when an 's' comes right before an unvoiced consonant like 't', it makes a "sh" sound. Therefore, este is pronounced like "esh-tuh".