Hi,
MIRO postings are related to MM and will be used to create Invoice, Credit Memo, Subsequent Debit/Credits, company code wise based on the Inbound Delivery created through t-code VL31N for the purchase orders. Subsequently Material Documents will be created in RBKP table and Accounting documents will be created in BKPF/BSEG tables. Please note that Goods Receipt can be done before/after the MIRO postings.
FB70 postings are related to FI to create Customer Invoices / Credit Memos basis the G/L accounts and we can see these documents through FB03 t-code and subsequent Accounting documents will be created in BKPF/BSEG tables.
Hope it helps.
BR
Gangadhar