Bet combines with hey if you want to write "in the" vs "in". You'd spell it the same. From COLLOQUIAL HEBREW:
Inseparable prepositions You may have noticed that unlike English the prepositions ‘to’ l’ , ‘from’ mi’ and ‘in’ b’ are single letters attached to the noun they precede. These are known as ‘inseparable prepositions’. NB An apostrophe will indicate an inseparable preposition in our transliteration. ‘mi’ becomes me’ before silent letters and some gutturals; e.g. me’oxford. You will however often hear Israelis using mi’ in all cases.
Inseparable prepositions + ‘the’ When the prepositions b’ and l’ are attached to definite nouns, ‘the’ is dropped and are pronounced ba and la. It is useful to show the vowels here in order to explain how this works (see vowel table on p. 14): Example: in an office (b’misrad) ; in the office (ba’misrad) Without vowels, the words look the same but you will know how to pronounce them correctly from the context.
NB Our third inseparable preposition ‘from’ mi’ does not contract when combined with ha’ : from the office me’ha’misrad Remember that proper names are definite by nature and do not take ‘the’ . The prepositions therefore do not change either. In Tel Aviv is b’tel-aviv . Composite names such as the Sheraton Hotel are also definite, are not preceded by in Hebrew, and the prepositions and do not change: in the Sheraton Hotel is b’malon sheraton
It literally means the same thing but in English* they have different context of when they are used more (I think). Inside of the restaurant is used when the focus of the sentence is your location. (But it's not used a lot in comparison to "in the restaurant.") Example: Where are you now? I'm inside OF Fancy (or the name of the restaurant) right now, near the back.
OR: I'm AT a restaurant right now, can I call you back?
Or: We're IN a restaurant, we'll be finished in an hour.
(This is East Coast (New York, Boston, Florida) American English.
As a preposition - בתוך, when you explicitly follow it with the surrounding thing. As an adverb - בפנים, when the surrounding thing is known in the context and not stated explicitly after the "inside". So if you take the sentence above, you might say in English "I am inside", where it's known in context that what you're inside of is their restaurant - Hebrew will say "אני בפנים".