@hope2019 Delighted to answer in detail and give as much time as needed. Let's start slow here
Haraam, a word rooted in limits/sanctity, a simple meaning is forbidden.
Only Allah decides what is Haram and what is halal (permissible).
On the outset many things are clearly Haram or halal, as they have been spelled out. And there is a spectrum between the these that are unclear, and we are told by the prophet to stay away from them. (That is when in doubt about issues permissibility, it's best to take another clear route that is known to be permissible/halal, and avoid getting into the details, scholars who have spent a lifetime can deduce (jurisprudence) a good understanding of such issues)
Now as for interest, Allah clearly says that he has forbidden usury and permitted trade. And dealing in interest is declaring war against Allah and his messenger. This is explicit.
Where people try to argue and stir the pot is what "forms" of usury/interest are forbidden. There were many forms of usury that were prevalent before the prohibition, and there is no explicit mention of allowing specific forms of usury by the prophet. In all, this alludes to having all forms of usury/interest being Haram.
There is one form that is not considered interest, it when a future payment in installments or full is agreed upon on a higher price (there is no compounding of interest). Buy car now at 10k, or pay me $12 within a year. (It's a cash for car)
Also an explicit hadith forbids the trade of like with like (trading one kind of dates with another kind of dates), which means you can't lend $x and have an extra fixed amount paid later $x+10 (it's money for money)
Also the hadith about partaking in interest, either you witness it, give it, take it, or write record it, all have committed the same sin.
PS - I'm no scholar, please consult an imam and get more info. I tried my best here.