Memory Upgrade Configuration

I iihave an HP laptop with 8 gb ram. It's a ryzen 5 4500u with 2 slots ,with 4gb ram each.
I want to upgrade my ram. Laptop specs says 8gb DDR 4 3200 mhz. They gave me the following specs to install.
DDR4-3200 dual-channel support DDR4-2400 dual-channel (DDR4-3200 bridge to DDR4-2666, DDR4-2666 downgrade to DDR-2400)
I dont understand the," DDR4-3200 bridge to DDR4-2666, DDR4-2666 downgrade to DDR-2400" What does it mean?
Supports the following configuration s
16 GB (8 × 2) ]
12 GB (8 + 4)
8 (4 × 2)
4 GB (4 × 1)
I have selected a module suggested by Crucial selection tool which recommends :
Unbuffered NON-ECC DDR4-3200 PC4-25600 CL=22 1.2V. 8 gb abnd plan to install 4 gb,+⁸gb)
Also HP says I can combine 8+4 gigs ram modules. can it be done. Is this ram compatible. Can I mix 8+4 modules. Wil there be any slow performance. Thanks
Hp Pavilion 15-eh0090wm
Ryzen 5 4500 U
8 gb DDR 8 Ram