Hi Sharma,
MB901 was meant to replace the former CRM introduction course in which we focussed only in CRM (Curtomer Engagement ie : Sales/Service and Marketing)). This course is supposed to give an overview of the Dynamics 365 stack of applications from Sales to Finances & Operations) but to me it was impossible to teach in two days time. That is why, I think, MS created two new courses MB910 and MB930.
PL900 on the other hand is an introduction course to Power Platform in wich we demonstrate/teach Dataverse, Canvas Apps, Model driven Apps, Power Automate , Power BI and Power Virtual agent but it is still an overview (2 days).
I think PL900 is the one you should start with. Then take PL100 and PL200.
Best regards.
Youcef Doumandji