Beleza....
Veja se é isso o que você precisa.
<!DOCTYPE html>
<html lang="pt-br">
<head>
<meta charset="UTF-8" />
<meta name="Author" content="" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<link rel="shortcut icon" href="./img/favicon.ico" />
<link rel="stylesheet" type="text/css" href="globalstyle.css" media="" />
<title>Projeto</title>
</head>
<body>
<!-- CREI UMA DIV GERAL-->
<div class="geral">
<div id="header"><?php require_once('header.php') ?></div>
<div id="content"><?php require_once('content.php') ?></div>
<div id="footer"><?php require_once('footer.php') ?></div>
</div>
</body>
</html>
---------------------------------------
CSS
html, body {height:100%;}
/*criei esse geral */
.geral {
min-height:100%;
position:relative;
width:100%;
}
header {
display: grid;
background: yellow;
}
.menu {
display: inline-block;
flex-wrap: wrap;
justify-content: space-between;
margin: 0 auto;
height: 50px;
max-width: 100%;
backdrop-filter: blur(0.5vh);
}
nav {
height: 100%;
flex-basis: calc(120% - 10vh);
display: flex;
text-align: center;
margin: 0 auto;
justify-content: center;
height: 100%;
}
.menu ul {
flex-basis: 100%;
display: flex;
justify-content: space-between;
align-items: center;
}
.menu li{
list-style-type:none;
}
.menu li a {
padding: 0.5vh 2vh;
color: #000;
text-decoration: none;
font-size: 12px;
opacity: 10;
transition: opacity 400ms;
}
.menu li a:hover {
opacity: 0.4;
text-decoration: none;
}
.link-logo {
background: url('logotipo-header.png');
background-repeat: no-repeat;
display: flex;
background-size: 3vh;
height: 5vh;
width: 3.5vh;
background-position: center;
}
.link-bag {
background: url('bag-header.png');
background-repeat: no-repeat;
display: flex;
background-size: 2vh;
height: 5vh;
width: 3.5vh;
background-position: center;
}
.link-search {
background: url('search-header.png');
background-repeat: no-repeat;
display: flex;
background-size: 2vh;
height: 5vh;
width: 3.5vh;
background-position: center;
opacity: 10;
}
content {display: grid;
grid-template-columns: 1fr;
grid-template-rows: 1fr;
grid-column-gap: 0px;
grid-row-gap: 0px;
background-color: darkred;
height: 100%;
font-size: 24px;}
/*alterei o position*/
footer {
bottom: 0;
background: rgba(0,0,0,0.8);
color: white;
width: 100%;
position: inherit;
}
footer a {color: #dcdcdc; opacity: 10; opacity: 0.8;transition: opacity 400ms;}
footer a:link{text-decoration: none; color: white;}
footer a:visible{text-decoration: none; color: white;}
footer a:hover{text-decoration: underline; color: white; opacity: 0.8;}
.infofooter {opacity: 0.8;
transition: opacity 400ms;}
.footer3 {
display: grid;
grid-template-columns: repeat(1, 1fr);
grid-gap: 2vh;
margin: 0 auto;
max-width: 175vh;
padding: 5vh;
align-items: center;
}
.footer3-item {
text-align: center;
max-height: 3vh;
}