Q1 - Get the customer whose spent increases every-month from 202103 - 202105
Q2 - Get the customer whose remain exists from 202103 - 202105
Q3 - Get the customer who stopped spent in 202104 and 202105
Q4 - Get each month level customers exists of which old customers VS Newly added customers VS customers who stopped spending
Multi row Formula would help to solve your task.
I prepared WF to answer to Q1 and Q2, so please finish the rest of questions by yourself with reference to my WF.
If you could give some context, explanation, greeting, etc., you would get more help. Please try to elaborate more from next time.