Over 100 junk emails per day

Getting 100+ junk emails a day does not mean Outlook/Microsoft shared your email address with a third party. Email addresses are commonly obtained through data breaches, websites you previously registered with, public webpages, compromised contact lists, or automated harvesting.

If the increase was very sudden. For example, you normally received a few junk messages and then suddenly started receiving hundreds of newsletter registrations, account confirmations, or messages in different languages, you should take it more seriously. This can be a spam bomb, also called subscription bombing. This actually happened to me a few years ago, but with my Gmail address at that time.

In a subscription/newsletter signup-bombing attack, someone enters your email address into large numbers of legitimate websites and mailing lists, often using an automated script. Those websites then send genuine registration or confirmation messages to your address. More importantly, sometimes the flood is intentionally created to hide one important email among hundreds of unimportant ones. In my case back then, it was because my CapitalOne credit card was compromised and was used by someone to make a big order at the Apple store. They were really hoping that the CapitalOne and Apple confirmation emails were hidden among the spam emails that suddenly flooded my inbox. Luckily I saw that. In fact, I also got a sudden increase of text messages confirming newsletter sign-ups from many sources too. Same, they were hoping I wouldn't then notice the alert CapitalOne sent me.

Therefore, if this started suddenly, I would not simply mass-delete everything. Search the messages carefully for terms such as order, purchase, payment, transaction, password, reset, security alert, verification, bank, PayPal, Amazon, Apple, Microsoft, and the names of any financial institutions or shopping accounts you use.

Such behavior is actually documented.. See the one shared by the Swiss Government: https://www.bacs.admin.ch/en/26w6-en

As for how to stop these emails. Unfortunately there's no easy way. You will have to unsubscribe each one using the link at the bottom of the email. It will take time for this to eventually die down. For my situation with Gmail, it took weeks for them to eventually go back to normal. Even as of today, I still occassionly received some random newsletter sign up confirmation emails.