Having not used an iPhone I'm not quite sure what you're asking. However, SMS/MMS text messaging is not like emails where your messages sit forever in an inbox that can call up and hit a 'reply' button. iMessages is not SMS/MMS messaging peer to peer (iPhone to iPhone), but will use the SMS/MMS messaging system to send messages to non-iPhone users (iPhone to Android).
I'm assuming you're using the stock messaging app (text)? The exact directions would vary depending on what keyboard you're using but I believe most are pretty similar in this regard. I'll address emojis. Emojis are right on your keyboard, usually shown with a smiley symbol, I know some of my older phones have had the smiley on the top that I'd tap to bring up emojis, then to return to the regular keyboard I believe there was a symbol showing two arrows encircling a "T", other keyboards I've had to go to the numbers/symbol screen to access emojis. Once you locate it, if you don't like the placement then you can customize your keyboard to what works best for you. Also you can quickly access stickers, gifs, memojis, camera, gallery etc, but again depends on which keyboard (Samsung, Google, Bixby etc). You can find your keyboard in settings under language and input, or you can access the settings right on the keyboard itself (there should be a settings wheel or possibly 3 dots) where you can see all of your options as well as the ways to customize the keyboard (including how you want to access Emojis) As far as replying to a specific text, one way is to find the text you want to reply to and hit "forward" & then select the same person you're texting & it'll take you to the text field where you'll see their text, you can simply type your reply above their text which is now in the text field. The other way, which gives you the same result and is just as easy/fast (faster in my opinion) is to simply copy the text you want to reply to & paste it into the text field, then type your message above it. I'll sometimes put the other person's text in quotes before typing my reply above it. It may not be as straight forward as iphone but trust me when I say that's a good thing! By doing things like I've described you'll stumble upon so much that you likely never knew was possible due to limitations on iPhones. I hope I was able to help, please forgive me if I haven't helped as this is my first time answering a question (I've used Samsung since the beginning & I'm ashamed to admit that I've only just now checked out Samsung Members🙈)