Hi ! first there is no shadda in marhaba, but yes in the majority of cases where you will see arabic texts, they won't be written with vowels (press and books ...) and yes we just intuitively know how to pronounce it, I would advise you to read as much as you can with vowels, so that you get familiar with arabic vocab, and then you will recongnize them by memory. Some words change vowels depending on their function in the sentence, if it's the case you just need to analyse the structure.

In arabic there is no vowels We have 3 diacritics They seem to be vowel but they are not The first on is فتحة its like this َ it is put on the top of the letter n its pronounced like schwa , the second diacritic is الضمة ُ its pronounced like this sound short /u/ , the final one is كسرة ِ its put under the letter its pronounced as this sound /I/ So marhaba we pronounce as this depending on these diacritics مَرحَبا I hope that u get it ^^

Hi! Here is my view. If this might help, I've been learning Arabic for 8 years and I can say now, I can really defend myself in the language. However, my experience is that: yes, it's like this. In daily writting language, vowels just disappear, but at the end, this is not a big deal since you're going to take it as normal. For those, words you already know, it'll become like intuitive and for those other you don't know, mostly, for its lexical constrution you'll know how to pronounce it. It's a matter of time :-) Anyways, Arabs, depending on which region they come from, they pronounce words differently. You know... Arabic language, at the same time it's beautiful, it's also amazingly rich in lexic.

The stress marks in Arabic aren't always written in normal books, magazines or newspapers, the only book you can always find them on is the Qura'an but I wouldn't recommend reading that as a beginner as it will frustrate you since the vocabulary used there has nothing to do with most of the vocabulary we use in normal dialects today. An interesting note that the original Arabic didn't even include the dots, so natives can actually understand the difference between T, TH, B without the dots because the meaning is already conveyed in the context and with experience we can identify what's the word. My best recommendation for you is to try to find a text with audio or to have someone read something out for you so you can get the right pronunciation from the first time and the wrong one doesn't stick and you would have to unlearn it. If you need any extra help with pronunciation you can message me! :)