Hi Danny,
Here is the link to the Lenovo service manual.
[link|https://pcsupport.lenovo.com/nz/en/products/laptops-and-netbooks/thinkbook-series/thinkbook-15-g3-acl/documentation/doc_userguide|Lenovo Thinkpad 15G3 ACL Service Manual|new_window=true]
As far as I know it is M.2 Pcie Nvme SSD
This video may also help.
[link|https://www.youtube.com/watch?v=fNVCMxYBqQE|Ungrading Lenovo Thinkpad 15 G3 ACL Laptop|new_window=true]
No storage size limitation.