You can do the same things in Calc as in Excel, but remember they are not the same applications.
Developers have defined keyboard shortcuts consistently according to the global design philosophy. In this case,
M is common to nearly all applications in the suite to mean Clear Direct Formatting, i.e. restore cell (selected element) to its base formatting defined by styles.
My best advice is to first force you to forget what you know about your previous suite workflow and read, at least superficially, the Calc Guide.
Outlook is a mailer outside LibreOffice suite. Your tags suggest you’ll use Thunderbird.
You can effectively “connect” LibreOffice to Thunderbird but I don’t recommend to do so if you’re not yet familiar with Linux (which distro?). It is as fast to have Thunderbird running in some window and to switch to it to create an email and attach your spreadsheet or other document.
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