レストランとかお寿司屋さんとかの本店とか西口店とかって英語でなんて言うの？

When a business such, as a restaurant chain or store chain has many locations, each location can be referred to as a branch of the business. It is normal to refer to each location as the "(name of location) branch" and this logic can used for all types of businesses on the high street such as banks, clothes stores etc. In the case of a small chain of stores, you could refer to each location as a sister store.

"The main store" means the biggest one. "The one in (name) street." This explains to the person which restaurant you are talking about by giving them the name of the street that it is in. "The restaurant opposite (name of Place)" This explains to the person where it is or which one it is by telling them what is on the other side of the road from it. "The one/restaurant by the west exit." This explains that it is the once by the west exit.

The Dominos on High Street. The McDonalds opposite the bank. These are ways which you can explain which store/restaurant you are referring to.

The flagship store or the headquarters would be what we'd use for a main location for a business.

The easiest way to explain where any outlet is in any town or city, is to mention a well=known landmark that is very near. The landmark could be a coffee shop, education center, library, church, McDonalds, car park or any other place. You will need to use an appropriate preposition of place such as: opposite, near, adjacent to, right next to, behind, looking over... etc.

If you are trying to describe what shop/place you are talking about it is better to explain where the shop is or where it is near to ( a landmark we call it - a well known place most people know) then the person would know how to find it and what shop/place you are talking about

Generally you would say the specific area the chain belongs to. For example Mcdonalds city centre. If it is widely known the store is the biggest or the newest you could use these adjectives as well.