Is Muay Thai similar to MMA?

Muay Thai is often part of MMA. I believe you've been misled to believe that anyone who fights in a cage trains in one Martial art that is called "MMA", which is simply untrue. MMA stands for "Mixed Martial Arts". MIXED. Meaning multiple martial arts styles are simultaneously used by one fighter. For example, MMA fighter #1 may train in Boxing, BJJ and Judo, while MMA fighter #2 may train in a different set of martial arts. If you're speaking about the sports themselves, MMA involves ground-fighting, submissions, throws and striking. Muay Thai is purely a striking art. Hope I answered your question.

No

Muay Thai is just a single style, the art which we called, "Art of Eight Limbs", some of its applications are punching, kicking, elbow, kneeing, clinching and neck wrestling.

MMA is a Mixed Martial Arts, it includes many applications, but things here rely already on a person if what are the styles covered by himself, it maybe a mix of muay thai, BJJ, wrestling, boxing and many more that a practitioner prefer to practice and trained.

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No.

Muay thai focuses on striking, grappling is limited to the stand-up clinch, you use boxing gloves for it.

MMA has striking and grappling, ground grappling and striking is a huge part of it, and you use 4oz gloves.