Using custom domain with a free mailbox

hello, how one can use a custom domain (eg. domain.tld) with a free mailbox from free email providers such as gmail,outlook,etc? on gmail for eg. it does not seems so simple as going to Settings>Accounts and Import>Add another email address because one have to enable "less secure apps", is this dangerous? there is other workaround such as changing DNS MX records of domain? as for email alias, how do i setup one? there is a better open secure clean email provider like gmail that one could use for free emailbox with a custom domain? thanks.