Inside if and else block you can use RedirectMatch directive as it works with it . RewriteRule doesn't work because it overrides and conflicts with IF/ELSE.

 RewriteEngine On

 <If "%{HTTP_HOST} == '<myHost>'">
 RedirectMatch 301 ^/$ https://example.com
 </If>
 <Else>
 RedirectMatch ^/$ https://example2.com
 </Else>

The <If> statement is not ignored. The "problem" is that when used inside an <If> expression the RewriteRule pattern matches the absolute filesystem path, not the requested URL-path that you are expecting.

The <If> block changes the order of processing. <If> blocks are merged very late, after .htaccess files are ordinarily processed and seemingly after the request has been remapped back to the filesystem (ie. after the directory-prefix has been added back).

With this in mind, you could change your rules to the following to get this to work:

<If "%{HTTP_HOST} == '<myHost>'">
  RewriteRule ^/file/path/to/document-root/$ <url> [R=301,L]
</If>
<Else>
  RewriteRule ^/file/path/to/document-root/$ <another_url> [R=301,L]
</Else>

Where /file/path/to/document-root/ is the absolute filesystem path to your document root directory (where the URL-path / maps to).

Alternatively, check the URL-path using the REQUEST_URI server variable in a condition instead. For example:

<If "%{HTTP_HOST} == '<myHost>'">
  RewriteCond %{REQUEST_URI} =/
  RewriteRule ^ <url> [R=301,L]
</If>
<Else>
  RewriteCond %{REQUEST_URI} =/
  RewriteRule ^ <another_url> [R=301,L]
</Else>

Or, check the REQUEST_URI var in the <If> (and <ElseIf>) expression(s). On Apache 2.4.26+ you can nest expressions. For example:

<If "%{REQUEST_URI} == '/'">
  <If "%{HTTP_HOST} == '<myHost>'">
    RewriteRule ^ <url> [R=301,L]
  </If>
  <Else>
    RewriteRule ^ <another_url> [R=301,L]
  </Else>
</If>

Or, use a mod_alias RedirectMatch directive inside the <If> expression, as suggested in @AmitVerma's answer. The mod_alias RedirectMatch (and Redirect) directives always reference the requested (root-relative) URL-path, regardless of context, so you don't get this ambiguity as with mod_rewrite.

Or, just use mod_rewrite, there is no need for the <If>/<Else> expression here. for example:

RewriteCond %{HTTP_HOST} =<myHost>
RewriteRule ^$ <url> [R=301,L]
RewriteRule ^$ <another_url> [R=301,L]