Yesterday, I was talking to my friend about smartphones.

Talking about them abstractly, you'd use the plural here. You could talk about "a smartphone", "my smartphone", "her smartphone", "the latest smartphone", but if you're not talking about a specific one then you are talking about smartphones.

She said in her case the most frequently used features in her smartphone is the torch.

Opposite problem here :) we're picking out one specific feature so we want the singular, not the plural. And we're modifying the verb "used", so we want the adverb "frequently" not the adjective "frequent".

I agree tood, because I used that a lot too.

Agreeing was a single thing you did, during this conversation which is in the past. So we put it in past tense. But using the torch is something you do habitually, you don't want to specify that you did it at any particular time. So simple present for that.

"I agree too" says that someone agrees with someone else, and you also agree. "Adam likes apples, Bob agrees with him, and I agree too." Not necessary here.

But she remindsed me that there is one feature in smartphone is not thas that not many people use.

Past tense again, abstract plural again.

In fact, I sawee many people still use a “real” calculator at work.

I can't quite explain why, but the simple past tense "saw" is strange here. "I see" again implies that this is something you see habitually, you have seen it before and you will again. But you could also say "I have seen many people still using".

I also usinge a calculator, even though my smartphone gothas the same function.

I wonder willif you havedo the same behavior just likething as me?

I think the grammar here is actually correct, but it's a very unnatural way to say this!

Do tell me if alsoyou're the same as me a. And can you explain why we didn’on't use that features in our smartphones?

"Do tell me" is an instruction, "can you explain" is a question, so I seperate them into two sentences. If they were both the same - "Do tell me if ... and tell me why ..." or "Can you tell me ... and can you explain ...?", then they could be in the same sentence.

Smartphones plural because you mean both yours and mine, at least two smartphones. Although if we were a couple and we shared a single phone, then you could say "in our smartphone".