What does warming up an inbox mean practically speaking ?

Hello community ,

I would like to ask experienced folks here something; does warming up an inbox involve many complications except to send/receive frequently , reply to threads , move mails from spam to main from recipients pow ?

I have a new inbox and Id like to test sending newsletters - guides to my app users but I need to be compliant and more importantly as a user It puts me off to see a mail in my spam so I have to make sure they wont.

I am a developer , worst case scenario I can automate this process but before thinking about the infrastructure , I want to ask about the numbers involved ; how many different inboxes it should send mails - receive from in this "warm up" period and is there a reliable method of testing if the mails land in spam , except for postmasters tools of google ?

Thank you