@michael.pearce, muita, as slang, is a variation of muito (very), which is an adverb. Adverbs are invariable words, so the standard word muito and the slang variation muita both retain their fixed form no matter what. Muita happens to look feminine, but in fact, it doesn’t really have a gender.
On the other hand, we also have the quantifiers muito, muita, muitos and muitas (many, a lot). These are variable words and will adjust accordingly. Muita as slang is not related to these.
- Aquele gajo é muita bacano. -> That dude is very cool. [slang adverb]
- Os filmes são muita bons! -> The movies are very good! [slang adverb]
- Há muitas opções. -> There are many options. [non-slang quantifier]
- Tenho muitos amigos. -> I have a lot of friends. [non-slang quantifier]
For other common slang terms, we have this detailed Learning Note (you’ve seen it, @michael.pearce, just leaving it here for reference): https://www.practiceportuguese.com/learning-notes/portuguese-slang/
We also had some fun on this other forum topic: How to say slangy phrases in Portuguese