When is something added to the message.mentions property?

[SteampunkWizardStudios]

There are several ways to mention a user with a bot, but it appears to me that it doesn't end up in the message.mentions property with my method.

The line which sends the message I want to get the mention from:
host.send({content: '${interaction.member} has requested to join as ${pjoin}', components: [row]});
${interaction.member} is replaced by <@discorduserid> in the message contents property.

The line which I'm using to access the mention:
dms.send('You denied ${message.mentions.users[0]} as some role');
The mentions.users array is empty even though the message is mentioning the user.

I'm pretty confused about this so thank you for your answers.

full list of properties of the message in question:

message: <ref *1> Message {
    channelId: '1059577974636159016',
    guildId: null,
    id: '1076217876526137445',
    createdTimestamp: 1676660736925,
    type: 0,
    system: false,
    content: '<@613438865084121124> has requested to join as P1',
    author: ClientUser {
      id: '1041824540080226435',
      bot: true,
      system: false,
      flags: [UserFlagsBitField],
      username: 'TDS Squads Bot',
      discriminator: '3914',
      avatar: 'dce90e9bdbd23fab6476b48d664f2408',
      banner: undefined,
      accentColor: undefined,
      verified: true,
      mfaEnabled: true
    },
    pinned: false,
    tts: false,
    nonce: null,
    embeds: [],
    components: [ [ActionRow] ],
    attachments: Collection(0) [Map] {},
    stickers: Collection(0) [Map] {},
    position: null,
    editedTimestamp: null,
    reactions: ReactionManager { message: [Circular *1] },
    mentions: MessageMentions {
      everyone: false,
      users: [Collection [Map]],
      roles: Collection(0) [Map] {},
      _members: null,
      _channels: null,
      _parsedUsers: null,
      crosspostedChannels: Collection(0) [Map] {},
      repliedUser: null
    },
    webhookId: null,
    groupActivityApplication: null,
    applicationId: null,
    activity: null,
    flags: MessageFlagsBitField { bitfield: 0 },
    reference: null,
    interaction: null
  },

[jaw0r3k]

this is collection
so you have to use .at(0) method to get the property

[SteampunkWizardStudios]

Thank you, I thought it was an array.