This is the feature that separates Atwork from a spreadsheet: sheets can point at each other, so a fact lives in exactly one place.

A Relation field links a record to one or more records in another sheet. Projects → a client. Tasks → a project. Invoices → a client.
Add a Relation field and choose the sheet it points at. Now each record can pick one (or several) records from there.
Why not just type the client's name? Because a typed name is a copy. Rename the client and every copy is wrong. A link stays correct, and lets you pull other facts through it.
A Mirror field displays a field from the linked record. Link a project to a client, then mirror the client's account manager or contract tier onto the project.
It is read-only and always current — change it on the client and every project reflects it.
A Rollup looks the other way: from the client, summarise the linked projects. Count them, sum their values, take the latest date.
So a Clients sheet can show number of open projects and total contract value per client without anyone maintaining those numbers.
Nothing is entered twice, and nothing goes stale.
Related: Build a sheet with custom fields · Formula fields