Suggested

Few new players

1) Question to the developers, is there an advertisement for the game now or is it planned only after the update?

2) Maybe it's time to start fighting small inactive leagues? For example, remove a league when there are less than five active players in the league (excluding private leagues) to make the leagues more competitive? Beginners enter the first league they come across, do not see the asset and leave. These leagues hurt the game.

As Jack once said, the game advertisement completely stopped in March 2021 and they haven't turned it on yet. They want to improve the game to a certain level where complaints, annoyances,... stopped before they decide to reopen marketing
I guess they will wait until the development blog is completed at least, or even wait until collisions, safety cars, flags... returned.
About the small inactive leagues though, it would still currently be a literal sandbox for new managers to explore the game. Its inactivity sometimes mean that managers will be able to experience domination in the league and found the game interesting.
In the future, small inactive leagues could be prevented by applying a "league making system" for new managers only, just like in the Top Eleven game.

Now there is no influx of new players, none at all. Old players are leaving one by one and new ones are not coming, all these updates can be released by the end of next year (although most of them should have been released according to the original plan a long time ago, but none have ever come out), and new players are needed already now, not in a year.

The smaller the asset, the sooner everyone will finally leave the game, there is no competition - there is no asset.

My understanding is that inactive leagues do get removed after a certain period.

New teams would generally join the top league on the league search when they join depending on the time they first login.

It would perhaps be better to fully explain the process of the game in terms of the race timings to the new teams, as just because you start your team at a certain time doesn’t mean that the time would be able to race at. 

Also I think it may be beneficial for rookie new joiners to see the top leagues with regards to activity rather than amount of teams. For example a league with 5 active players who have logged in within the past day would be more beneficial than a league with 28 teams who have not logged in since joining.

As a host of a new league, there is a steady stream of new joiners as long as the host knows the mechanics of the game for new joiners ??

I am one of the newer kind when it comes to this game. A friend got me to start playing. The thing is the rookie leauge is to easy to dominate most of the time. It is only when you have active players it becomes a bit more challenging. 

The game also has some problems that drive newer players away. An example of this is the fact that I can't buy stuff on auction for some reason. Idk why and idk how to find the reason and/or contact the devs.

Well, I also experienced a lot of these problems on auction. Sometimes you just can't press the 'bid' button. The reason behind this tends to be devs' upgrade on the system, or fixes, etc