Match the DDR generation (e.g., DDR4 or DDR5). These are not backward-compatible; a DDR5 slot will not fit a DDR4 stick.
You can buy "Dell-certified" memory or compatible third-party options. Laptop Memory Upgrades - Dell buy ram for dell laptop
Laptops use SO-DIMM (small outline) modules, which are shorter than desktop DIMMs. Match the DDR generation (e
It is best to match the speed of your existing RAM. If you mix speeds, the system will default to the slowest stick's speed. Match the DDR generation (e.g.