Could anyone explain to me how it works for a company to buy an open source app? Are they just buying the name and brand recognition? Is there anything to stop original developers to use the old source code and continue developing their own fork under a different name? Couldn't anyone else do the same? Do companies just buy the apps and pull the code from everywhere they can and make it closed source? Couldn't the company have just modified the freely available code without paying the original devs anything?
They get the play store account that published the apps, likely also some support from the original developer for a while.
The original play store account has all the downloads and reputation associated with the project. It also has the revenue from adds and new purchases. The developer can modify the apps and send these out as updates. This could include things like more ads and tracking to generate additional revenue.
If you honestly think that the F-Droid releases will ever contain ads and trackers, you have absolutely no clue what F-Droid is and how it works and you quickly need to educate yourself on the F-Droid website.
While I think it's more likely the apps will just never be updated on F-droid, ads and trackers are allowed on f-droid so that doesn't preclude them putting updated versions on F-droid.