I wanted to ask if you can make a server on which Minecraft Pocket Edition (on mobile phone) players and Minecraft Java Edition (on computer) players can play at the same time. Is that possible?
No. The two versions are fundamentally different and the content alone is so different that this is not possible. A server can only host one version of the game at a time.
Not really.
However, there's a proxy for the Java server that Bedrock players can leave on the server.
But this is highly experimental and then some gameplay-technical things change for the Bedrock players (because some things work differently in the Java version)
No, unfortunately that will not work.
Unfortunately, the Java edition players remain among themselves. Crossplay only works with the Windows 10 edition of the game.
Here is the project that is trying.
As I said. Highly experimental!