> Are there filters between each server?

There _might_ be filters. Mastodon server admins can filter posts from certain foreign servers. For example the metalhead.club administrator chose to filter some instances which spam a lot or post extremist text and /or child pornography.

Often these filters are necessary for legal reasons, e.g. some jananese servers allow sketches of naked children ("loli"), which are considered child pornography in Germany and are therefore illegal in the server's home country.