Drupal 9, write 2 functions

Pls read the project, before you answer.

Only Drupal 9 experts.

Function 1 :
We have an entity type bookings. This entity type ia attached as a field in a node. So the node have a booking form. This booking form have a select list field A.
The node have 4 select fields. Field B, C, D and E.
If there values from field B, C, D and E must be get to fill the options in select list A.
So you need to get the values from the node to put them in the form field that is attached in the node to fill the options.

Function 2
Validation in the node, that the user must select minimum 1 option from field B, C, D or E

I explain more in detail if you are able to write this.