Learn Portuguese (Portugal): O cruzamento tem muito trânsito.

No, it is a bit of a false friend. In Portuguese, trânsito means traffic (the vehicles on the road). If you want to talk about public transit, you would say transportes públicos.

I sometimes get confused about when muito changes to muita or muitos. Why is it exactly muito here?

Here, muito means 'a lot of', describing the noun trânsito. Because trânsito is a masculine singular word, muito stays in its default masculine singular form. If the sentence was about many cars (masculine plural), it would change to match: muitos carros.

Could I use há (there is) instead of tem (has) to talk about traffic here?

Yes, but the sentence structure would change. You could say Há muito trânsito no cruzamento (There is a lot of traffic at the crossroads). The current sentence says the crossroads itself has the traffic, which is a very natural way to phrase it in Portuguese.

What is a good trick to remember the word cruzamento?