How I can get current user information? You can use Get Current User block to get the user model, filled with current user details.
What is _with variables for? Why some blocks have them and some not? _with input variable is a system enum, that tells if need to get data from relations (if there any relations between models). By default _with is not set and you will get only IDs of records from related data models. Every enabled _with increases the overall load of your application because of increasing database requests to get additional data. In genera, _with works as classic JOIN operation for Relational DBs.
Is every placed on the canvas block have to be connected with Flow connections? In most cases - yes. Some blocks can be temporary excluded from generation by excluding them from Flow. All connected variables to this blow will return zero or null values.
How I can pass ID parameter from Endpoint Routes like /order/:id/ to BP? To get ID parameter passed from Endpoint to BP Start block, the Start block must have an input parameter with the same name (id). At this point, we support only one parameter per endpoint route.
How many blocks I can create in one business process? There is no actual limitation on the number of blocks, but we recommend limiting the overall number of blocks to 50 per business process. A good practice is to create a separate business process to implement frequently used business login and call them inside of your business process.