So a company isn’t allowed to advertise its other products?

More like, a company that is in the business of offering you information should not alter that information based on personal interest.

You wouldn't expect a map created by a company to hide streets where rival businesses are located for example, or your doctor to avoid disclosing a potential treatment because it's done by a rival health provider.

Funnily enough, I remember this being one of the differentiating factors that made people love google when they were starting to become popular - "our ads are unintrusive and clearly separated from actual content".

At tech giant's scale, I'd say yes. YouTube gets the level of promotion and integration with Google services that money can't buy.

Google and Microsoft use their successful products to promote and keep competition out for other products.

Microsoft got slapped for Internet Explorer, but somehow Google gets away with shoving Chrome and the Android certification program which is basically "you either bundle all our other products, or we'll make you incompatible with Play Services".

On the same platform where they sell ads to competing products? No.