Get certain values from a JSON file using PowerShell

I've seen a lot of questions about JSON and PowerShell these past hours and none helped me find a solution to this particular problem. And I'm sure it's something easy.

I want to extract all the url fields of the plugins objects in this JSON object (original URL is this: https://updates.jenkins.io/update-center.json):

{
    "connectionCheckUrl": "http://www.google.com/",
    "core": {
        ...
    },
    "deprecations": {
        ...
    },
    "generationTimestamp": "2021-05-19T12:16:52Z",
    "id": "default",
    "plugins": {
        "42crunch-security-audit": {
            "buildDate": "Oct 06, 2020",
            "defaultBranch": "master",
            "dependencies": [
                ...
            ],
            "developers": [
                ...
            ],
            "excerpt": "Performs API contract security audit to get a detailed analysis of the possible vulnerabilities and other issues in the API contract.",
            "gav": "io.jenkins.plugins:42crunch-security-audit:3.8",
            "issueTrackers": [
                ...
            ],
            "labels": [
                ...
            ],
            ...
            "title": "42Crunch REST API Static Security Testing",
            "url": "http://archives.jenkins-ci.org/plugins/42crunch-security-audit/3.8/42crunch-security-audit.hpi",
        },
        "AnchorChain": {
            ...
            "url": "http://archives.jenkins-ci.org/plugins/AnchorChain/1.0/AnchorChain.hpi",
            ...
        },
        ... many hundreds more ...
    }
    ...
}

The plugins object contains one object per plugin, where the plugin's name is the object's key. So I somehow have to iterate over all plugin objects and look for the url property.

I want/have to do this using PowerShell (v5.1) but cannot find an easy way. Here is where I am stuck:

$all = (Get-Content(".\update-center.json") | convertfrom-json)
$all.gettype().fullname

$plugins = $all.plugins
$plugins.gettype().fullname

I get this result:

System.Management.Automation.PSCustomObject
System.Management.Automation.PSCustomObject

And now I hope to iterate over the individual plugin objects and simply get the url key's property, but I'm stuck:

$plugins | get-member -MemberType NoteProperty | foreach name | foreach $plugins.$_.url

The get-member is supposed to get the individual plugins I suppose, but hours of poring over PowerShell documentation have clearly fried my brain. Help! :-)