A question about starters

Do you think that starter pokemons are generally better than pokemons of the same type?
Do you always keep them throughout the game or replace them if necessary?

I think that they are, "stacked" and expected to be better than Pokemon of the same type, however if I had to place them on a tier-based list, i'd say that they are middle-tier generally.

I generally ditch them early on myself, as I want a challenge, and find the starters to be less than fun to use.

Welcome to the community, Gummy :)
Starters are like the first best friends for me, so I generally prefer to keep them throughout the gameplay. They would be placed, in my opinion, in the lower sections of the top tiers or in the middle tiers. Without starters, it really gets a bit difficult, so if you want a challenge, do go for no starter run. But to have a Pokemon that most of the other trainers do not have is something cool. What about you? What do you think of them?

Usually! Often they're quite good and easily usable throughout the entire game without being replaced by others of the same type (designed to be that way since they're your first Pokémon), plus a lot of them end up with unique type combinations and good movesets. Meganium is one of the only exceptions since its movepool is quite poor and it doesn't do well against the many gyms in Johto that resist grass. I usually keep them, it just feels right aha.

Starters are designed to be used for the entire game, and I've always found them to be pretty well-balanced for that. Some are better than others, but I think a lot of what makes a starter good or bad is what the first few Gyms are like...as great as Charmander is, it's pretty damn useless until you get past Cerulean Gym. Poor Chikorita gets mauled in a lot of Gyms too. I usually keep mine in my party, although I ditched it in Gen V because aesthetically I couldn't stand any of them, and I don't like using ugly Pokemon.

I tend to keep them in the squad and would use them as me main fire/water/grass pokemon whether or not they are the best of their type im not sure

It depends on the starter, I think. Venusaur is undoubtedly the best grass type in Kanto, for example, but I can think of better water and fire types than blastoise and charizard in terms of stats, movepool and their usefulness. But I tend to always keep them in my party because they're my first pokémon — the only exceptions are nuzlockes and my one X playthrough where I got rid of my greninja (but kept my charizard) because it just wasn't pulling its weight.

Not necessarily but one thing is sure, they're very strong for the story. They're basically designed to be useful during all the stages of the game.
If I not using my starter it's always right off the bat, I never replaced it mid or late game.

It depends, like there are actually stronger Electric types than the Pikachu starter (Except the one in LGP though in term of usefulness).

On the Yellow version the only starter I used was Venusaur; There are better Water and Fire I could use instead of Blastoise and Charizard.

I usually do a playthrough with every starter. I like to do themed runs so if the starter fits a theme I want to do, I might not pick him the first time around so I can pick him for my theme run (eg Rowlet for an all birds theme). If neither starter fits the theme or if I've already used them all, I might box my starter