Which way should I connect our inboxes?

There are three doors, and the right one depends on who owns the mailboxes. Pick from this table and follow the linked article.

1. 1Your company runs Google Workspace or Microsoft 365 and you want the whole sales team connected: use Connect your whole team. One approval from your IT admin, nobody signs in, new hires are a tick box. Takes IT about two minutes.
2. 2You bought inboxes from a mailbox provider (Litemail, Mailforge, Mailreef, Zapmail, Premium Inboxes, or an agency reselling them) and the domains are not yours to administer: upload the provider's file. Settings → Integrations → Connect → Upload from your provider (CSV). Each inbox verifies itself in the background. Most of these providers give you Google Workspace inboxes, so the file needs an app password per inbox — see the guide.
3. 3It is just you, or a personal Gmail or Outlook.com address: sign in yourself. Settings → Integrations → Connect → Google (Gmail) or Microsoft (Outlook).
4. 4Not sure who administers your domain? The person who set up your work email, or whoever adds new starters, is the admin. Send them the link Monty gives you and they will know.

You can mix them. A team can have thirty inboxes through the admin approval, ten bought inboxes from a CSV, and one founder's personal Gmail — the plan's mailbox count is what limits the total.