if()
{
  query = query.Where(....)
}

if()
{
   query = query.Where(....)
}

query = query.Where(....).Where(....);

if(condition)
{
   predicate.And(something);
}
if(otherCondition)
{
   predicate.Or(somethingElse);
}
var result = source.Where(predicate);

var flg1 = cond1 == null? true : cond1;
var flg2 = cond2 == null? true : cond2;

var result = query.Where(flg1 && flg2)

query.Where("field1=1 and field2=2");