It's all up to the relay operators. Operators choose what note kinds to allow (or not), how to store them, how long, whether to respect deletion requests, etc.
Different relay operators set up different infrastructures. Some are free, some are paid, some are invite-only. Some relay operators have distributed nodes under the same domain name to provide optimized response times by routing you to a node near you geographically. Some relays even sync and share data with others to better reflect the overall state of Nostr's network.
The relays are quickly becoming a rich landscape, and they are, IMO, one of the most exciting parts of protocol development right now. nostr:nprofile1qqs99d9qw67th0wr5xh05de4s9k0wjvnkxudkgptq8yg83vtulad30gpp4mhxue69uhkummn9ekx7mqpz3mhxue69uhhyetvv9ujuerpd46hxtnfduq32amnwvaz7tmgd9ehgtnwdaehgu3wd3skueqyluj3x, nostr:nprofile1qqs8eseg5zxak2hal8umuaa7laxgxjyll9uhyxp86c522shn9gj8crspz9mhxue69uhkummnw3ezuamfdejj7qgjwaehxw309ahx7um5wgerztnrdakj7qgkwaehxw309a3x2an09ehx7um5wgcjucm0d5hsvlnggv, and nostr:nprofile1qqsyeqqz27jc32pgf8gynqtu90d2mxztykj94k0kmttxu37nk3lrktcpz9mhxue69uhkummnw3ezuamfdejj7qg3waehxw309ahx7um5wghxcctwvshsz9nhwden5te0d4kx26m49eex2ctv0yhxcmmv9ume3twc are all doing exciting work in this space.