concurrency in running query

Question

I am using Postgres12 and DBeaver. I insert thousands of data using a procedure like this :

do $$
declare
    ctr_max_ID integer;
    ctr_loop Integer := 1;
begin 
    loop
    exit when ctr_loop = 5001; 
    ctr_loop := ctr_loop + 1;  
    select max (cast(id as Integer)) into ctr_max_ID from table1 ;
    insert into table1 (id, data) values (ctr_max_ID + 1, 'DUMMY data');
    raise notice 'ctr_loop: %', ctr_loop;
    end loop;
end; $$;

While that procedure running, I issued another select statement "Select * From table1 Where ID = '1';" This statement is never executed until the procedure finish running. I also run a statement for another table "Select * From table2 Limit 1". This is not executed until stored procedure finish running.

Is this normal ? Or is there some setting somewhere that specifies query has to run one at a time ?

thanks