The spoken form has vowels. The written form has them in the form of dots and lines that go around the letters (which represent consonant sounds, not whole words), but they're usually omitted other than in kids books.

For example, the word for friend is "חָבֵר" (chaver), but you'd usually see it as "חבר". It's written right to left, so if you look at the first letter, the ח is the "ch" sound and the Tetris-piece-looking thing below it is the "a". These are called "dots".

In practice, you get used to it pretty easily as a reader. Instead of "chaver", it's "chvr", but reading it still becomes second nature.

Consider that English only has 5-6 written vowels (depending on if you count "y"), but 14-15 different vowel sounds in American English. So, how do you know if an "o" is the sound in "owl" or the one in "owe" or the one in "on"? You just learn it when you learn to read. Same thing with Hebrew vowels.

There are some letters that seem vowel-y (like א, which is roughly like an "ah" sound), for historical reasons. What they actually are are consonants whose pronunciation is either subtle or silent (depending on the letter, and one's accent/pronunciation) and which are almost always associated with a particular vowel sound.