If you wonder why "restaurant" is a loan word, well, there are some Japanese words for similar meaning, like
料理店（りょうりてん）/料理屋（りょうりや）, literally a store (店)/ a house (屋) of cuisine(料理)
食堂（しょくどう）, literally a hall (堂) for eating (食) , more like a cafeteria.
食事処（しょくじどころ）, literally a place (処) for meal (食事).
Actually there are many words related to "meal", and many words similar to "a house," and they can even be combined to form a noun similar to "restaurant" of something like that. But, if you find any of these in Japan, basically they all sell Japanese cuisine. Western restaurants are just called レストラン.
I try to remember that it's "backwards," so to speak. It's pronounced "restaraunt" and then it's spelled "restaurant." Also, you can imagine how infuriating the spelling is, and remember the last part of the word is "rant."
Now restaurant doesn't look like a word anymore haha.
The Japanese alphabet system is decided into 3 main sections
Hiragana - basic alphabet for native words
Katakana - standard alphabet for all foreign words
Kanji - text taken from China most of which share similarity, over 1900 commonly used kanji but there are WAY more.
There is also
Romanji - basically it's spelling out the Japanese words using Roman letters
It would be written, in romaji/romanji, as roomaji to indicate the long vowel, but English != romaji. The pronunciation rules of romaji do not match the pronunciation rules of English. For example, repeating a vowel in English would suggest a different sound (o vs. oo), whereas in Romaji the spelling would reflect the Japanese spelling.