@media (max-width: 575px) {
    #img{
      display: none;
    }

    .tamanho{
      min-width: 100%;
      min-height: 100%;
  }

   body{
    font-size: 60%;
  }
}


body{
  margin: 0;
  padding: 0;
  font-size: 100%;
}

.colorlink {
    color: grey;
}

.colorlink:hover {
    transition: 1000ms;
    color: black;
}

.pracima {
    right: 90%;
    bottom: 90%;
}

.colorcopy {
    color: gray;
}


header {
  position: relative;
  background-color: black;
  height: 75vh;
  min-height: 25rem;
  width: 100%;
  overflow: hidden;
}

header video {
  position: absolute;
  top: 50%;
  left: 50%;
  min-width: 100%;
  min-height: 100%;
  width: auto;
  height: auto;
  z-index: 0;
  -ms-transform: translateX(-50%) translateY(-50%);
  -moz-transform: translateX(-50%) translateY(-50%);
  -webkit-transform: translateX(-50%) translateY(-50%);
  transform: translateX(-50%) translateY(-50%);
}

header .container {
  position: relative;
  z-index: 2;
}

header .overlay {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  background-color: black;
  opacity: 0.5;
  z-index: 1;
}

/* Media Query for devices withi coarse pointers and no hover functionality */

/* This will use a fallback image instead of a video for devices that commonly do not support the HTML5 video element */

Edit in JSFiddle
Result
HTML
CSS
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Resources
header {
  position: relative;

  background-color: black;
  height: 75vh;
  min-height: 25rem;
  width: 100%;
  overflow: hidden;
}

header video {
  position: absolute;
  top: 50%;
  left: 50%;
  min-width: 100%;
  min-height: 100%;
  width: auto;
  height: auto;
  z-index: 0;
  -ms-transform: translateX(-50%) translateY(-50%);
  -moz-transform: translateX(-50%) translateY(-50%);
  -webkit-transform: translateX(-50%) translateY(-50%);
  transform: translateX(-50%) translateY(-50%);
}

header .container {
  position: relative;
  z-index: 2;
}

header .overlay {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  background-color: black;
  opacity: 0.5;
  z-index: 1;
}

/* Media Query for devices withi coarse pointers and no hover functionality */

/* This will use a fallback image instead of a video for devices that commonly do not support the HTML5 video element */

/* @media (pointer: coarse) and (hover: none) {
  header {
    background: url('https://source.unsplash.com/XT5OInaElMw/1600x900') black no-repeat center center scroll;
  }

  header video {
    display: none;
  }
} */

.back {
  background-image: url(/img/purple.jfif);
  opacity: 0.7;
  background-size:100% 100%;
  background-repeat: no-repeat;
}

.botao {
  padding: 0.8px;
  border: 1px solid rgb(252, 252, 252);
  border-radius: 6px;
  background-color: rgb(247, 247, 247);
}

.botao:hover {
  transition: 430ms;
  background-color: rgb(237, 237, 237);
}